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{{short description|Network that allows computers to share resources and communicate with each other}}
{{Redirect|Datacom}}
{{Other uses|Network (disambiguation){{!}}Network}}
{{more citations needed|date=June 2023}}
{{Use American English|date=April 2020}}

{{Network science}}
{{OS}}

A '''computer network''' is a collection of communicating [[computer]]s and other devices, such as printers and smart phones. Today almost all computers are connected to a computer network, such as the global [[Internet]] or an embedded network such as those found in modern cars. Many applications have only limited functionality unless they are connected to a computer network. Early computers had very limited connections to other devices, but perhaps the first example of computer networking occurred in 1940 when [[George Stibitz]] connected a terminal at Dartmouth to his Complex Number Calculator at [[Bell Labs]] in New York. 

In order to communicate, the computers and devices must be connected by a physical medium that supports transmission of information. A variety of technologies have been developed for the physical medium, including wired media like copper cables and [[optical fibers]] and wireless [[radio-frequency]] media. The computers may be connected to the media in a variety of [[network topologies]]. In order to communicate over the network, computers use agreed-on rules, called [[communication protocol]]s, over whatever medium is used.<ref>{{cite book |last1=Peterson |first1=Larry | last2=Davie | first2=Bruce |date=2000 |title=Computer Networks: A Systems Approach |url=https://book.systemsapproach.org/index.html |location=Singapore |publisher=Harcourt Asia  |isbn= 9789814066433 |access-date=May 24, 2025}}</ref><ref>{{cite book |last=Anniss |first=Matthew |date=2015 |title=Understanding Computer Networks |location=United States |publisher=Capstone |isbn= 9781484609071}}</ref> 

The computer network can include [[personal computer]]s, [[Server (computing)|servers]], [[networking hardware]], or other specialized or general-purpose [[Host (network)|hosts]]. They are identified by [[network address]]es and may have [[hostname]]s. Hostnames serve as memorable labels for the nodes and are rarely changed after initial assignment. Network addresses serve for locating and identifying the nodes by communication protocols such as the [[Internet Protocol]].

Computer networks may be classified by many criteria, including the [[transmission medium]] used to carry signals, [[bandwidth (computing)|bandwidth]], [[communications protocol]]s to organize [[network traffic]], the network size, the topology, [[Network traffic control|traffic control]] mechanisms, and organizational intent.{{Citation needed|date=January 2023}}

Computer networks support many [[Application software|applications]] and [[Network service|services]], such as access to the [[World Wide Web]], [[digital video]] and [[digital audio|audio]], shared use of [[File server|application and storage servers]], [[Printer (computing)|printers]] and [[fax machine]]s, and use of email and instant messaging applications.

== History ==
Computer networking may be considered a branch of [[computer science]], [[computer engineering]], and [[telecommunications]], since it relies on the theoretical and practical application of the related disciplines. Computer networking was influenced by a wide array of technological developments and historical milestones.
* In 1940, [[George Stibitz]] of [[Bell Labs]] connected a teletype at Dartmouth to a Bell Labs computer running his Complex Number Calculator to demonstrate the use of computers at long distance.<ref>{{cite book|last=Ritchie |first=David|date=1986|chapter=George Stibitz and the Bell Computers |title=The Computer Pioneers|url=https://archive.org/details/computerpioneers00ritc |url-access=registration |page=[https://archive.org/details/computerpioneers00ritc/page/35 35]|location=New York|publisher=Simon and Schuster|isbn=067152397X}}</ref><ref>{{Cite book |url=https://books.google.com/books?id=AsvSBQAAQBAJ&q=%22Complex+computer%22+1939&pg=PA481 |title=History of Computing in the Twentieth Century |last=Metropolis |first=Nicholas |date=2014 |publisher=Elsevier |isbn=9781483296685 |pages=481 |language=en}}</ref> This was the first real-time, remote use of a computing machine.<ref>{{cite book|last=Ritchie |first=David|date=1986|chapter=George Stibitz and the Bell Computers |title=The Computer Pioneers|url=https://archive.org/details/computerpioneers00ritc |url-access=registration |page=[https://archive.org/details/computerpioneers00ritc/page/35 35]|location=New York|publisher=Simon and Schuster|isbn=067152397X}}</ref>
* In the late 1950s, a network of computers was built for the U.S. military [[Semi-Automatic Ground Environment]] (SAGE) [[radar]] system<ref>{{cite book|url=https://books.google.com/books?id=RBC2nY1rp5MC&dq=semi+automatic+ground+environment+network&pg=PA399|title=Military Communications: From Ancient Times to the 21st Century|year=2008 |editor-first=Christopher H.|editor-last=Sterling|publisher=[[ABC-Clio]]|isbn=978-1-85109-737-1|page=399}}</ref><ref>{{cite book|url=https://books.google.com/books?id=KOY9EAAAQBAJ&dq=semi+automatic+ground+environment+network&pg=PA87|title=A New History of Modern Computing|first1=Thomas|last1=Haigh|first2=Paul E.|last2=Ceruzzi|date=14 September 2021 |author-link2=Paul E. Ceruzzi|publisher=[[MIT Press]]|isbn=978-0262542906|pages=87–89}}</ref><ref>{{cite book|url=https://books.google.com/books?id=ZITpBQAAQBAJ&q=semi+automatic+ground+environment|title=AN/FSQ-7: the computer that shaped the Cold War|first=Bernd|last=Ulmann|publisher=[[De Gruyter]]|date=August 19, 2014|isbn=978-3-486-85670-5}}</ref> using the [[Bell 101 modem]]. It was the first commercial [[modem]] for computers, released by [[AT&T Corporation]] in 1958. The modem allowed [[digital data]] to be transmitted over regular unconditioned telephone lines at a speed of 110 bits per second (bit/s).
* In 1959, [[Christopher Strachey]] filed a patent application for [[time-sharing]] in the United Kingdom and [[John McCarthy (computer scientist)|John McCarthy]] initiated the first project to implement time-sharing of user programs at MIT.<ref name="ctsspg3">{{cite book|first=F. J.|last=Corbató|author-link=Fernando J. Corbató|display-authors=et al.|url=http://www.bitsavers.org/pdf/mit/ctss/CTSS_ProgrammersGuide.pdf|title=The Compatible Time-Sharing System A Programmer's Guide]|publisher=MIT Press|year=1963|isbn=978-0-262-03008-3|quote=Shortly after the first paper on time-shared computers by C. Strachey at the June 1959 UNESCO Information Processing conference, H. M. Teager and J. McCarthy at MIT delivered an unpublished paper "Time-shared Program Testing" at the August 1959 ACM Meeting.|access-date=2020-05-26|archive-date=2012-05-27|archive-url=https://web.archive.org/web/20120527174321/http://www.bitsavers.org/pdf/mit/ctss/CTSS_ProgrammersGuide.pdf|url-status=live}}</ref><ref>{{Cite web|title=Computer Pioneers - Christopher Strachey|url=https://history.computer.org/pioneers/strachey.html|access-date=2020-01-23|website=history.computer.org|archive-date=2019-05-15|archive-url=https://web.archive.org/web/20190515062531/https://history.computer.org/pioneers/strachey.html|url-status=live}}</ref><ref>{{Cite web|title=Reminiscences on the Theory of Time-Sharing|url=https://jmc.stanford.edu/computing-science/timesharing.html|access-date=2020-01-23|website=jmc.stanford.edu|archive-date=2020-04-28|archive-url=https://web.archive.org/web/20200428094453/http://jmc.stanford.edu/computing-science/timesharing.html|url-status=dead}}</ref><ref>{{Cite web|title=Computer - Time-sharing and minicomputers|url=https://www.britannica.com/technology/computer|access-date=2020-01-23|website=Encyclopedia Britannica|language=en|archive-date=2015-01-02|archive-url=https://web.archive.org/web/20150102192452/http://www.britannica.com/EBchecked/topic/130429/computer/216032/Invention-of-the-modern-computer|url-status=live}}</ref> Strachey passed the concept on to [[J. C. R. Licklider]] at the inaugural [[International Federation for Information Processing#History|UNESCO Information Processing Conference]] in Paris that year.<ref>{{Cite book|last1=Gillies|first1=James M.|url=https://archive.org/details/howwebwasbornsto00gill|title=How the Web was Born: The Story of the World Wide Web|last2=Gillies|first2=James|last3=Gillies|first3=James and Cailliau Robert|last4=Cailliau|first4=R.|date=2000|publisher=Oxford University Press|isbn=978-0-19-286207-5|pages=[https://archive.org/details/howwebwasbornsto00gill/page/13 13]|language=en|url-access=registration}}</ref> McCarthy was instrumental in the creation of three of the earliest time-sharing systems (the [[Compatible Time-Sharing System]] in 1961, the [[BBN Time-Sharing System]] in 1962, and the [[Dartmouth Time-Sharing System]] in 1963).
* In 1959, [[Anatoly Kitov]] proposed to the Central Committee of the Communist Party of the Soviet Union a detailed plan for the re-organization of the control of the Soviet armed forces and of the Soviet economy on the basis of a network of computing centers.<ref>{{Cite web|last=Kitova|first=O|others=Translated by Alexander Nitusov|title=Kitov Anatoliy Ivanovich. Russian Virtual Computer Museum|url=https://www.computer-museum.ru/english/galglory_en/Kitov.htm|access-date=2021-10-11|website=computer-museum.ru|archive-date=2023-02-04|archive-url=https://web.archive.org/web/20230204143509/https://www.computer-museum.ru/english/galglory_en/Kitov.htm|url-status=live}}</ref> Kitov's proposal was rejected, as later was the 1962 [[OGAS]] economy management network project.<ref>{{Cite book |isbn = 978-0262034180|title = How Not to Network a Nation: The Uneasy History of the Soviet Internet|last1 = Peters|first1 = Benjamin|date = 25 March 2016| publisher=MIT Press }}</ref>
* In 1960, the commercial airline reservation system [[semi-automatic business research environment]] (SABRE) went online with two connected [[mainframe]]s.
* In 1963, J. C. R. Licklider sent a memorandum to office colleagues discussing the concept of the "[[Intergalactic Computer Network]]", a computer network intended to allow general communications among computer users.
* In 1965, [[Western Electric]] introduced the first widely used [[telephone switch]] that implemented computer control in the switching fabric.
* Throughout the 1960s,<ref name=":17">{{Cite journal |last=Baran |first=Paul |date=2002 |title=The beginnings of packet switching: some underlying concepts |url=http://web.cs.ucla.edu/~lixia/papers/Baran2002.pdf |url-status=live |journal=IEEE Communications Magazine |language=en |volume=40 |issue=7 |pages=42–48 |doi=10.1109/MCOM.2002.1018006 |issn=0163-6804 |archive-url=https://ghostarchive.org/archive/20221010/http://web.cs.ucla.edu/~lixia/papers/Baran2002.pdf |archive-date=2022-10-10 |quote=Essentially all the work was defined by 1961, and fleshed out and put into formal written form in 1962. The idea of hot potato routing dates from late 1960.}}</ref><ref name=":23">{{cite journal |last=Roberts |first=Lawrence G. |date=November 1978 |title=The evolution of packet switching |url=http://www.ece.ucf.edu/~yuksem/teaching/nae/reading/1978-roberts.pdf |journal=Proceedings of the IEEE |volume=66 |issue=11 |pages=1307–13 |doi=10.1109/PROC.1978.11141 |s2cid=26876676 |quote=Almost immediately after the 1965 meeting, Davies conceived of the details of a store-and-forward packet switching system. |issn=0018-9219 }}</ref> [[Paul Baran]] and [[Donald Davies]] independently invented the concept of [[packet switching]] for [[data communication]] between computers over a network.<ref>{{cite book|last1=Isaacson|first1=Walter|url=https://books.google.com/books?id=4V9koAEACAAJ&pg=PA245|title=The Innovators: How a Group of Hackers, Geniuses, and Geeks Created the Digital Revolution|date=2014|publisher=Simon and Schuster|isbn=9781476708690|pages=237–246|access-date=2021-06-04|archive-date=2023-02-04|archive-url=https://web.archive.org/web/20230204143508/https://books.google.com/books?id=4V9koAEACAAJ&pg=PA245|url-status=live}}</ref><ref name=":232">{{cite journal |last=Roberts |first=Lawrence G. |date=November 1978 |title=The evolution of packet switching |url=https://www.ece.ucf.edu/~yuksem/teaching/nae/reading/1978-roberts.pdf |url-status=live |journal=Proceedings of the IEEE |volume=66 |issue=11 |pages=1307–13 |doi=10.1109/PROC.1978.11141 |s2cid=26876676 |archive-url=https://web.archive.org/web/20230204143508/https://www.ece.ucf.edu/~yuksem/teaching/nae/reading/1978-roberts.pdf |archive-date=2023-02-04 |access-date=2022-02-12 |quote=Both Paul Baran and Donald Davies in their original papers anticipated the use of T1 trunks}}</ref><ref>{{cite web|title=NIHF Inductee Paul Baran, Who Invented Packet Switching|url=https://www.invent.org/inductees/paul-baran|access-date=2022-02-12|publisher=National Inventors Hall of Fame|archive-date=2022-02-12|archive-url=https://web.archive.org/web/20220212204116/https://www.invent.org/inductees/paul-baran|url-status=live}}</ref><ref>{{cite web|title=NIHF Inductee Donald Davies, Who Invented Packet Switching|url=https://www.invent.org/inductees/donald-watts-davies|access-date=2022-02-12|publisher=National Inventors Hall of Fame|archive-date=2022-02-12|archive-url=https://web.archive.org/web/20220212204117/https://www.invent.org/inductees/donald-watts-davies|url-status=live}}</ref> Baran's work addressed adaptive routing of message blocks across a distributed network, but did not include routers with software switches, nor the idea that users, rather than the network itself, would provide the [[Reliability (computer networking)|reliability]].<ref>{{Cite journal |last=Baran |first=P. |date=1964 |title=On Distributed Communications Networks |url=https://ieeexplore.ieee.org/document/1088883 |journal=IEEE Transactions on Communications |language=en |volume=12 |issue=1 |pages=1–9 |doi=10.1109/TCOM.1964.1088883 |issn=0096-2244|url-access=subscription }}</ref><ref>{{Cite journal |last=Kleinrock |first=L. |date=1978 |title=Principles and lessons in packet communications |url=https://ieeexplore.ieee.org/document/1455412 |journal=Proceedings of the IEEE |volume=66 |issue=11 |pages=1320–1329 |doi=10.1109/PROC.1978.11143 |issn=0018-9219 |quote=Paul Baran ... focused on the routing procedures and on the survivability of distributed communication systems in a hostile environment, but did not concentrate on the need for resource sharing in its form as we now understand it; indeed, the concept of a software switch was not present in his work.|url-access=subscription }}</ref><ref name="Pelkey6.1a">{{Cite book |last=Pelkey |first=James L. |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications 1968–1988 |chapter=6.1 The Communications Subnet: BBN 1969 |quote=As Kahn recalls: ... Paul Baran's contributions ... I also think Paul was motivated almost entirely by voice considerations. If you look at what he wrote, he was talking about switches that were low-cost electronics. The idea of putting powerful computers in these locations hadn't quite occurred to him as being cost effective. So the idea of computer switches was missing. The whole notion of protocols didn't exist at that time. And the idea of computer-to-computer communications was really a secondary concern. |chapter-url=https://historyofcomputercommunications.info/section/6.1/the-communications-subnet-bbn-1969/}}</ref><ref name=":5a">{{Cite book |last=Waldrop |first=M. Mitchell |url=https://books.google.com/books?id=eRnBEAAAQBAJ&pg=PT285 |title=The Dream Machine |date=2018 |publisher=Stripe Press |isbn=978-1-953953-36-0 |pages=286 |language=en |quote=Baran had put more emphasis on digital voice communications than on computer communications.}}</ref> Davies' hierarchical network design included high-speed [[Router (computing)|routers]], [[communication protocol]]s and the essence of the [[end-to-end principle]].<ref name=":6">{{Cite book |last=Yates |first=David M. |url=https://books.google.com/books?id=ToMfAQAAIAAJ&q=packet+switch |title=Turing's Legacy: A History of Computing at the National Physical Laboratory 1945-1995 |date=1997 |publisher=National Museum of Science and Industry |isbn=978-0-901805-94-2 |pages=132–4 |language=en |quote=Davies's invention of packet switching and design of computer communication networks ... were a cornerstone of the development which led to the Internet}}</ref><ref>{{Cite book |last=Naughton |first=John |author-link=John Naughton |url=https://archive.org/details/briefhistoryoffu0000naug/page/292/mode/2up |title=A Brief History of the Future |date=2000 |publisher=Phoenix |isbn=9780753810934 |page=292 |language=en |orig-date=1999}}</ref><ref name=":3">{{Cite journal |last=Campbell-Kelly |first=Martin |date=1987 |title=Data Communications at the National Physical Laboratory (1965-1975) |url=https://archive.org/details/DataCommunicationsAtTheNationalPhysicalLaboratory |journal=Annals of the History of Computing |language=en |volume=9 |issue=3/4 |pages=221–247 |doi=10.1109/MAHC.1987.10023 |s2cid=8172150 |quote=the first occurrence in print of the term protocol in a data communications context ... the next hardware tasks were the detailed design of the interface between the terminal devices and the switching computer, and the arrangements to secure reliable transmission of packets of data over the high-speed lines}}</ref><ref name=":5">{{cite conference |last1=Davies |first1=Donald |last2=Bartlett |first2=Keith |last3=Scantlebury |first3=Roger |last4=Wilkinson |first4=Peter |date=October 1967 |title=A Digital Communication Network for Computers Giving Rapid Response at remote Terminals |url=https://people.mpi-sws.org/~gummadi/teaching/sp07/sys_seminar/how_did_erope_blow_this_vision.pdf |conference=ACM Symposium on Operating Systems Principles |archive-url=https://ghostarchive.org/archive/20221010/https://people.mpi-sws.org/~gummadi/teaching/sp07/sys_seminar/how_did_erope_blow_this_vision.pdf |archive-date=2022-10-10 |access-date=2020-09-15 |url-status=live}} "all users of the network will provide themselves with some kind of error control"</ref> The [[NPL network]], a [[local area network]] at the [[National Physical Laboratory (United Kingdom)]], pioneered the implementation of the concept in 1968-69 using {{nowrap|768 kbit/s}} links.<ref>{{cite conference |last=Scantlebury |first=R. A. |author2=Wilkinson, P.T. |year=1974 |title=The National Physical Laboratory Data Communications Network |url=http://www.rogerdmoore.ca/PS/NPLPh/NPL1974A.html |pages=223–228 |book-title=Proceedings of the 2nd ICCC 74}}</ref><ref name=":3" /><ref>{{Cite news|author=Guardian Staff|date=2013-06-25|title=Internet pioneers airbrushed from history|language=en-GB|work=The Guardian|url=https://www.theguardian.com/technology/2013/jun/25/internet-pioneers-airbrushed-from-history|access-date=2020-07-31|issn=0261-3077|quote=This was the first digital local network in the world to use packet switching and high-speed links.|archive-date=2020-01-01|archive-url=https://web.archive.org/web/20200101062943/https://www.theguardian.com/technology/2013/jun/25/internet-pioneers-airbrushed-from-history|url-status=live}}</ref> Both Baran's and Davies' inventions were seminal contributions that influenced the development of computer networks.<ref name=":16">{{Cite news |title=The real story of how the Internet became so vulnerable |url=http://www.washingtonpost.com/sf/business/2015/05/30/net-of-insecurity-part-1/ |url-status=dead |archive-url=https://web.archive.org/web/20150530231409/http://www.washingtonpost.com/sf/business/2015/05/30/net-of-insecurity-part-1/ |archive-date=2015-05-30 |access-date=2020-02-18 |newspaper=Washington Post |language=en-US |quote=Historians credit seminal insights to Welsh scientist Donald W. Davies and American engineer Paul Baran}}</ref><ref name=":24">{{cite journal |last1=Roberts |first1=Lawrence G. |date=November 1978 |title=The Evolution of Packet Switching |url=http://www.ismlab.usf.edu/dcom/Ch10_Roberts_EvolutionPacketSwitching_IEEE_1978.pdf |url-status=dead |journal=IEEE Invited Paper |archive-url=https://web.archive.org/web/20181231092936/http://www.ismlab.usf.edu/dcom/Ch10_Roberts_EvolutionPacketSwitching_IEEE_1978.pdf |archive-date=31 December 2018 |access-date=September 10, 2017 |quote=In nearly all respects, Davies' original proposal, developed in late 1965, was similar to the actual networks being built today.}}</ref><ref name=":13">{{Cite book |last1=Norberg |first1=Arthur L. |title=Transforming computer technology: information processing for the Pentagon, 1962-1986 |last2=O'Neill |first2=Judy E. |date=1996 |publisher=Johns Hopkins Univ. Press |isbn=978-0-8018-5152-0 |series=Johns Hopkins studies in the history of technology New series |location=Baltimore |pages=153–196}} Prominently cites Baran and Davies as sources of inspiration.</ref><ref>{{cite report |url=https://apps.dtic.mil/sti/pdfs/ADA115440.pdf |title=A History of the ARPANET: The First Decade |date=1 April 1981 |publisher=Bolt, Beranek & Newman Inc. |pages=13, 53 of 183 (III-11 on the printed copy) |quote=Aside from the technical problems of interconnecting computers with communications circuits, the notion of computer networks had been considered in a number of places from a theoretical point of view. Of particular note was work done by Paul Baran and others at the Rand Corporation in a study "On Distributed Communications" in the early 1960's. Also of note was work done by Donald Davies and others at the National Physical Laboratory in England in the mid-1960's. ... Another early major network development which affected development of the ARPANET was undertaken at the National Physical Laboratory in Middlesex, England, under the leadership of D. W. Davies. |archive-url=https://web.archive.org/web/20121201013642/http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc.pdf&AD=ADA115440 |archive-date=1 December 2012 |url-status=live}}</ref>
* In 1969, the first four nodes of the [[ARPANET]] were connected using {{nowrap|50 kbit/s}} circuits between the University of California at Los Angeles, the Stanford Research Institute, the University of California at Santa Barbara, and the University of Utah.<ref>{{cite web|author=Chris Sutton|title=Internet Began 35 Years Ago at UCLA with First Message Ever Sent Between Two Computers|url=https://www.engineer.ucla.edu/stories/2004/Internet35.htm|archive-url=https://web.archive.org/web/20080308120314/https://www.engineer.ucla.edu/stories/2004/Internet35.htm|archive-date=2008-03-08|url-status=dead|publisher=[[UCLA]]}}</ref> Designed principally by [[Bob Kahn]], the network's routing, flow control, software design and network control were developed by the [[Interface Message Processor|IMP]] team working for [[Bolt Beranek & Newman]].<ref name=":2322">{{cite journal |last=Roberts |first=Lawrence G. |date=November 1978 |title=The evolution of packet switching |url=http://www.ece.ucf.edu/~yuksem/teaching/nae/reading/1978-roberts.pdf |journal=Proceedings of the IEEE |volume=66 |issue=11 |pages=1307–13 |doi=10.1109/PROC.1978.11141 |s2cid=26876676 |quote=Significant aspects of the network's internal operation, such as routing, flow control, software design, and network control were developed by a BBN team consisting of Frank Heart, Robert Kahn, Severo Omstein, William Crowther, and David Walden}}</ref><ref name="F.E. Froehlich, A. Kent2">{{cite book |author=F.E. Froehlich, A. Kent |url=https://books.google.com/books?id=gaRBTHdUKmgC&pg=PA344 |title=The Froehlich/Kent Encyclopedia of Telecommunications: Volume 1 - Access Charges in the U.S.A. to Basics of Digital Communications |date=1990 |publisher=CRC Press |isbn=0824729005 |page=344 |quote=Although there was considerable technical interchange between the NPL group and those who designed and implemented the ARPANET, the NPL Data Network effort appears to have had little fundamental impact on the design of ARPANET. Such major aspects of the NPL Data Network design as the standard network interface, the routing algorithm, and the software structure of the switching node were largely ignored by the ARPANET designers. There is no doubt, however, that in many less fundamental ways the NPL Data Network had and effect on the design and evolution of the ARPANET.}}</ref><ref>{{cite tech report|last1=Heart|first1=F.|last2=McKenzie|first2=A.|last3=McQuillian|first3=J.|last4=Walden|first4=D.| url=https://walden-family.com/bbn/arpanet-completion-report.pdf | archive-url=https://web.archive.org/web/20230527095942/https://walden-family.com/bbn/arpanet-completion-report.pdf | archive-date=2023-05-27 | url-status=dead|title=Arpanet Completion Report|publisher=Bolt, Beranek and Newman|location=Burlington, MA|date=January 4, 1978}}</ref> In the early 1970s, [[Leonard Kleinrock]] carried out mathematical work to model the performance of packet-switched networks, which underpinned the development of the ARPANET.<ref name=":8">{{Cite thesis |last=Clarke |first=Peter |title=Packet and circuit-switched data networks |date=1982 |degree=PhD |publisher=Department of Electrical Engineering, Imperial College of Science and Technology, University of London |url=https://spiral.imperial.ac.uk/bitstream/10044/1/35864/2/Clarke-PN-1982-PhD-Thesis.pdf}} "Many of the theoretical studies of the performance and design of the ARPA Network were developments of earlier work by Kleinrock ... Although these works concerned message switching networks, they were the basis for a lot of the ARPA network investigations ... The intention of the work of Kleinrock [in 1961] was to analyse the performance of store and forward networks, using as the primary performance measure the average message delay. ... Kleinrock [in 1970] extended the theoretical approaches of [his 1961 work] to the early ARPA network."</ref><ref name=":4">{{Cite book |last=Davies |first=Donald Watts |url=https://archive.org/details/computernetworks00davi/page/86/mode/2up?q=kleinrock+kleinrock%27s |title=Computer networks and their protocols |date=1979 |publisher=Wiley |others=Internet Archive |isbn=978-0-471-99750-4 |pages=See page refs highlighted at url |quote=In mathematical modelling use is made of the theories of queueing processes and of flows in networks, describing the performance of the network in a set of equations. ... The analytic method has been used with success by Kleinrock and others, but only if important simplifying assumptions are made. ... It is heartening in Kleinrock's work to see the good correspondence achieved between the results of analytic methods and those of simulation.}}</ref> His theoretical work on [[hierarchical routing]] in the late 1970s with student [[Farouk Kamoun]] remains critical to the operation of the Internet today.<ref name=":52">{{Cite book |last=Davies |first=Donald Watts |url=https://archive.org/details/computernetworks00davi/page/110/mode/2up?q=%22kleinrock+and+kamoun%22 |title=Computer networks and their protocols |date=1979 |publisher=Wiley |others=Internet Archive |isbn=978-0-471-99750-4 |pages=110–111 |quote=Hierarchical addressing systems for network routing have been proposed by Fultz and, in greater detail, by McQuillan. A recent very full analysis may be found in Kleinrock and Kamoun.}}</ref><ref>{{Cite book |last1=Feldmann |first1=Anja |title=Proceedings of the 2009 workshop on Re-architecting the internet |last2=Cittadini |first2=Luca |last3=Mühlbauer |first3=Wolfgang |last4=Bush |first4=Randy |last5=Maennel |first5=Olaf |date=2009 |publisher=Association for Computing Machinery |isbn=978-1-60558-749-3 |series=ReArch '09 |location=New York, NY, USA |pages=43–48 |chapter=HAIR: Hierarchical architecture for internet routing |doi=10.1145/1658978.1658990 |quote=The hierarchical approach is further motivated by theoretical results (e.g., [16]) which show that, by optimally placing separators, i.e., elements that connect levels in the hierarchy, tremendous gain can be achieved in terms of both routing table size and update message churn. ... [16] KLEINROCK, L., AND KAMOUN, F. Hierarchical routing for large networks: Performance evaluation and optimization. Computer Networks (1977). |chapter-url=https://core.ac.uk/download/pdf/326320693.pdf |s2cid=2930578}}</ref>
* In 1972, commercial services were first deployed on experimental [[public data network]]s in Europe.<ref>{{Cite web |last=Derek Barber |title=The Origins of Packet Switching |url=http://www.cs.man.ac.uk/CCS/res/res05.htm#f |access-date=2024-06-05 |website=Computer Resurrection Issue 5 |quote=The Spanish, dark horses, were the first people to have a public network. They'd got a bank network which they craftily turned into a public network overnight, and beat everybody to the post.}}</ref><ref>{{cite conference |last=Després |first=R. |author-link=Rémi Després |year=1974 |title=RCP, the Experimental Packet-Switched Data Transmission Service of the French PTT |url=http://rogerdmoore.ca/PS/RCPDEP/RD.html |conference= |pages=171–185 |archive-url=https://web.archive.org/web/20131020142207/http://rogerdmoore.ca/PS/RCPDEP/RD.html |archive-date=2013-10-20 |access-date=2013-08-30 |book-title=Proceedings of ICCC 74 |url-status=dead}}</ref>
* In 1973, the French [[CYCLADES]] network, directed by [[Louis Pouzin]] was the first to make the hosts responsible for the reliable delivery of data, rather than this being a centralized service of the network itself.<ref>{{cite web|last1=Bennett|first1=Richard|title=Designed for Change: End-to-End Arguments, Internet Innovation, and the Net Neutrality Debate|url=https://www.itif.org/files/2009-designed-for-change.pdf|publisher=Information Technology and Innovation Foundation|access-date=2017-09-11|page=11|date=September 2009|archive-date=2019-08-29|archive-url=https://web.archive.org/web/20190829092926/http://www.itif.org/files/2009-designed-for-change.pdf|url-status=dead}}</ref>
* In 1973, [[Peter T. Kirstein|Peter Kirstein]] put [[internetworking]] into practice at [[University College London]] (UCL), connecting the ARPANET to [[Internet in the United Kingdom#History|British academic networks]], the first international heterogeneous computer network.<ref name="Early experiences with the Arpanet">{{cite journal |last1=Kirstein |first1=P.T. |date=1999 |title=Early experiences with the Arpanet and Internet in the United Kingdom |journal=IEEE Annals of the History of Computing |volume=21 |issue=1 |pages=38–44 |doi=10.1109/85.759368 |s2cid=1558618}}</ref><ref>{{Cite journal |last=Kirstein |first=Peter T. |date=2009 |title=The early history of packet switching in the UK |journal=IEEE Communications Magazine |volume=47 |issue=2 |pages=18–26 |doi=10.1109/MCOM.2009.4785372 |s2cid=34735326}}</ref>
* In 1973, [[Robert Metcalfe]] wrote a formal memo at [[Xerox PARC]] describing [[Ethernet]],<ref>{{Cite web |title=Xerox Researcher Proposes 'Ethernet' |url=https://www.computerhistory.org/tdih/may/22/ |access-date=2025-03-08 |website=computerhistory.org |quote=Robert Metcalfe, a researcher at the Xerox Palo Alto Research Center in California, writes his original memo proposing an 'Ethernet', a means of connecting computers together.}}</ref> a local area networking system he created with [[David Boggs]].<ref>{{Cite web |title=Ethernet is Still Going Strong After 50 Years - IEEE Spectrum |url=https://spectrum.ieee.org/ethernet-ieee-milestone |access-date=2025-03-08 |website=spectrum.ieee.org |language=en}}</ref> It was inspired by the [[packet radio]] [[ALOHAnet]], started by [[Norman Abramson]] and [[Franklin F. Kuo|Franklin Kuo]] at the University of Hawaii in the late 1960s.<ref>{{Cite web |title=ALOHAnet – University of Hawai'i College of Engineering |url=https://www.eng.hawaii.edu/about/history/alohanet/ |access-date=2025-03-08 |website=eng.hawaii.edu |language=en-US}}</ref><ref>{{Cite web |title=Celebrating 50 Years of the ALOHA System and the Future of Networking |url=https://www.computerhistory.org/collections/catalog/102792095 |access-date=2025-03-08 |website=computerhistory.org}}</ref> Metcalfe and Boggs, with [[John Shoch]] and Edward Taft, also developed the [[PARC Universal Packet]] for internetworking.<ref>{{Cite web |last=Hsu |first=Hansen |last2=McJones |first2=Paul |title=Xerox PARC file system archive |url=https://xeroxparcarchive.computerhistory.org/Xerox_PARC_source_code.html |access-date= |website=xeroxparcarchive.computerhistory.org |language=en |quote=Pup (PARC Universal Packet) was a set of internetworking protocols and packet format designed and first implemented (in BCPL) by David R. Boggs, John F. Shoch, Edward A. Taft, and Robert M. Metcalfe. It became a key influence on the later design of TCP/IP.}}</ref>
* In 1974, [[Vint Cerf]] and [[Bob Kahn]] published their seminal 1974 paper on internetworking, ''A Protocol for Packet Network Intercommunication.<ref name=":0">{{Cite journal |last1=Cerf |first1=V. |last2=Kahn |first2=R. |date=1974 |title=A Protocol for Packet Network Intercommunication |url=https://www.cs.princeton.edu/courses/archive/fall06/cos561/papers/cerf74.pdf |journal=IEEE Transactions on Communications |volume=22 |issue=5 |pages=637–648 |doi=10.1109/TCOM.1974.1092259 |issn=1558-0857 |quote=The authors wish to thank a number of colleagues for helpful comments during early discussions of international network protocols, especially R. Metcalfe, R. Scantlebury, D. Walden, and H. Zimmerman; D. Davies and L. Pouzin who constructively commented on the fragmentation and accounting issues; and S. Crocker who commented on the creation and destruction of associations.}}</ref>'' Later that year, Cerf, [[Yogen Dalal]], and Carl Sunshine wrote the first [[Transmission Control Protocol]] (TCP) specification, {{IETF RFC|675}}, coining the term ''Internet'' as a shorthand for internetworking.<ref>{{cite IETF |title=Specification of Internet Transmission Control Protocol |rfc=675 |last1=Cerf |first1=Vinton|last2=dalal|first2=Yogen|last3=Sunshine|first3=Carl |date=December 1974 |publisher=[[IETF]]}}</ref>
* In July 1976, Metcalfe and Boggs published their paper "Ethernet: Distributed Packet Switching for Local Computer Networks"<ref>{{cite journal |author1=Robert M. Metcalfe |author2=David R. Boggs |date=July 1976 |title=Ethernet: Distributed Packet Switching for Local Computer Networks |journal=Communications of the ACM |volume=19 |issue=5 |pages=395–404 |doi=10.1145/360248.360253 |s2cid=429216 |doi-access=free}}</ref> and in December 1977, together with [[Butler Lampson]] and [[Charles P. Thacker]], they received {{US patent|4063220A}} for their invention.<ref>{{Cite web |last=Press |first=Gil |title=The Ethernet And The Telegraph Or What Metcalfe And Morse Have Wrought |url=https://www.forbes.com/sites/gilpress/2017/05/22/the-ethernet-and-the-telegraph-or-what-metcalfe-and-morse-have-wrought/ |access-date=2025-03-08 |website=Forbes |language=en}}</ref><ref>{{Cite web |title=Ethernet and Robert Metcalfe and Xerox PARC 1971-1975 {{!}} History of Computer Communications |url=https://historyofcomputercommunications.info/section/8.7/Ethernet-and-Robert-Metcalfe-and-Xerox-PARC-1971-1975/ |access-date=2025-03-08 |website=historyofcomputercommunications.info |quote=Once successful, Xerox filed for patents covering the Ethernet technology under the names of Metcalfe, Boggs, Butler Lampson and Chuck Thacker. (Metcalfe insisted Lampson, the 'intellectual guru under whom we all had the privilege to work' and Thacker 'the guy who designed the Altos' names were on the patent.)}}</ref>
* [[Public data network]]s in Europe, North America and Japan began using [[X.25]] in the late 1970s and interconnected with [[X.75]].<ref name=":232" /> This underlying infrastructure was used for expanding TCP/IP networks in the 1980s.<ref>{{Cite book |last1=Council |first1=National Research |url=https://books.google.com/books?id=Jh1pORpfvrQC&pg=PA148 |title=The Unpredictable Certainty: White Papers |last2=Sciences |first2=Division on Engineering and Physical |last3=Board |first3=Computer Science and Telecommunications |last4=Applications |first4=Commission on Physical Sciences, Mathematics, and |last5=Committee |first5=NII 2000 Steering |date=1998-02-05 |publisher=National Academies Press |isbn=978-0-309-17414-5 |language=en |access-date=2021-03-08 |archive-url=https://web.archive.org/web/20230204143512/https://books.google.com/books?id=Jh1pORpfvrQC&pg=PA148 |archive-date=2023-02-04 |url-status=live}}</ref>
* In 1976, John Murphy of [[Datapoint Corporation]] created [[ARCNET]], a token-passing network first used to share storage devices.
* In 1977, the first long-distance fiber network was deployed by GTE in Long Beach, California.
* In 1979, Robert Metcalfe pursued making Ethernet an open standard.<ref name="Spurgeon 2000">{{cite book|last=Spurgeon|first=Charles E.|title=Ethernet The Definitive Guide|url=https://archive.org/details/ethernetdefiniti0000spur|url-access=registration|year=2000|publisher=O'Reilly & Associates|isbn=1-56592-660-9}}</ref>
* In 1980, Ethernet was upgraded from the original {{nowrap|2.94 Mbit/s}} protocol to the 10&nbsp;Mbit/s protocol, which was developed by [[Ron Crane (engineer)|Ron Crane]], Bob Garner, Roy Ogus,<ref>{{Cite web|url=https://www.wband.com/2013/05/introduction-to-ethernet-technologies/|title=Introduction to Ethernet Technologies|publisher=WideBand Products|website=www.wband.com|language=en-US|access-date=2018-04-09|archive-date=2018-04-10|archive-url=https://web.archive.org/web/20180410072256/https://www.wband.com/2013/05/introduction-to-ethernet-technologies/|url-status=live}}</ref> and Yogen Dalal.<ref name="Pelkey-Dalal">{{cite book |last1=Pelkey |first1=James L. |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications, 1968-1988 |date=2007 |chapter=Yogen Dalal |chapter-url=https://historyofcomputercommunications.info/interviews/yogen-dalal/ |access-date=2023-05-07}}</ref>
* In 1995, the transmission speed capacity for Ethernet increased from 10&nbsp;Mbit/s to 100&nbsp;Mbit/s. By 1998, Ethernet supported transmission speeds of 1&nbsp;Gbit/s. Subsequently, higher speeds of up to 800&nbsp;Gbit/s were added ({{as of | 2025 | lc = on}}). The scaling of Ethernet has been a contributing factor to its continued use.<ref name="Spurgeon 2000"/>
* In the 1980s and 1990s, as [[embedded systems]] were becoming increasingly important in factories, cars, and airplanes, [[network protocols]] were developed to allow the embedded computers to communicate. In the late 1990s and 2000s, [[ubiquitous computing]] and an [[Internet of Things]] became popular.<ref>{{cite web |author=<!-- not stated --> |date=November 26, 2019 |title=What is the Importance of Embedded Networking? |url=https://www.totalphase.com/blog/2019/11/what-is-importance-of-embedded-networking/ |website=Total Phase|location=United States |publisher=Total Phase, Inc. |access-date=June 23, 2025}}</ref><ref>{{cite journal |last1= Gershenfeld |first1=Neil |last2= Krikorian |first2=Raffi | last3=Cohen | first3=Danny |date=October 2004 |title=The internet of things |url=https://www.jstor.org/stable/26060727?seq=1 |journal=Scientific American |volume=291 |issue=4 |publisher=Springer Nature |pages=76-81 |access-date=June 23, 2025}}</ref>

==Use==
Computer networks enhance how users communicate with each other by using various electronic methods like email, instant messaging, online chat, voice and video calls, and video conferencing. Networks also enable the sharing of computing resources. For example, a user can print a document on a shared printer or use shared storage devices. Additionally, networks allow for the sharing of files and information, giving authorized users access to data stored on other computers. [[Distributed computing]] leverages resources from multiple computers across a network to perform tasks collaboratively.

==Network packet==
[[File:Network packet.jpg|thumb|Network Packet]]
Most modern computer networks use protocols based on packet-mode transmission. A [[network packet]] is a formatted unit of [[data]] carried by a [[packet-switched network]].

Packets consist of two types of data: control information and user data (payload). The control information provides data the network needs to deliver the user data, for example, source and destination [[network address]]es, [[error detection]] codes, and sequencing information. Typically, control information is found in [[Header (computing)|packet headers]] and [[Trailer (computing)|trailers]], with [[payload data]] in between.

With packets, the [[Bandwidth (computing)|bandwidth]] of the transmission medium can be better shared among users than if the network were [[circuit switched]]. When one user is not sending packets, the link can be filled with packets from other users, and so the cost can be shared, with relatively little interference, provided the link is not overused. Often the route a packet needs to take through a network is not immediately available. In that case, the packet is [[message queue|queued]] and waits until a link is free.

The physical link technologies of packet networks typically limit the size of packets to a certain [[maximum transmission unit]] (MTU). A longer message may be fragmented before it is transferred and once the packets arrive, they are reassembled to construct the original message.

==Network topology==
{{further|Network topology}}
[[File:NetworkTopologies.svg|thumb|right|Common network topologies]]
The physical or geographic locations of network nodes and links generally have relatively little effect on a network, but the topology of interconnections of a network can significantly affect its throughput and reliability. With many technologies, such as bus or star networks, a single failure can cause the network to fail entirely. In general, the more interconnections there are, the more robust the network is; but the more expensive it is to install. Therefore, most [[network diagram]]s are arranged by their [[network topology]] which is the map of logical interconnections of network hosts.

Common topologies are:
* [[Bus network]]: all nodes are connected to a common medium along this medium. This was the layout used in the original [[Ethernet]], called [[10BASE5]] and [[10BASE2]]. This is still a common topology on the [[data link layer]], although modern [[physical layer]] variants use [[point-to-point (telecommunications)|point-to-point]] links instead, forming a star or a tree.
* [[Star network]]: all nodes are connected to a special central node. This is the typical layout found in a small [[switched Ethernet]] LAN, where each client connects to a central network switch, and logically in a [[wireless LAN]], where each wireless client associates with the central [[wireless access point]].
* [[Ring network]]: each node is connected to its left and right neighbor node, such that all nodes are connected and that each node can reach each other node by traversing nodes left- or rightwards. [[Token ring]] networks, and the [[Fiber Distributed Data Interface]] (FDDI), made use of such a topology.
* [[Mesh network]]: each node is connected to an arbitrary number of neighbors in such a way that there is at least one traversal from any node to any other.
* [[Fully connected network]]: each node is connected to every other node in the network.
* [[Tree network]]: nodes are arranged hierarchically. This is the natural topology for a larger Ethernet network with multiple switches and without redundant meshing.

The physical layout of the nodes in a network may not necessarily reflect the network topology. As an example, with [[FDDI]], the network topology is a ring, but the physical topology is often a star, because all neighboring connections can be routed via a central physical location. Physical layout is not completely irrelevant, however, as common ducting and equipment locations can represent single points of failure due to issues like fires, power failures and flooding.

===Overlay network===
[[File:Network Overlay merged.svg|thumb|upright=1.5|A sample overlay network]]
An [[overlay network]] is a virtual network that is built on top of another network. Nodes in the overlay network are connected by virtual or logical links. Each link corresponds to a path, perhaps through many physical links, in the underlying network. The topology of the overlay network may (and often does) differ from that of the underlying one. For example, many [[peer-to-peer]] networks are overlay networks. They are organized as nodes of a virtual system of links that run on top of the [[Internet]].<ref name=over>{{citation |author1=D. Andersen |author2=H. Balakrishnan |author3=M. Kaashoek |author4-link=Robert Tappan Morris |author4=R. Morris |url=http://nms.lcs.mit.edu/papers/ron-sosp2001.html |title=Resilient Overlay Networks |date=October 2001 |publisher=[[Association for Computing Machinery]] |access-date=2011-11-12 |archive-date=2011-11-24 |archive-url=https://web.archive.org/web/20111124125227/http://nms.lcs.mit.edu/papers/ron-sosp2001.html |url-status=live }}</ref>

Overlay networks have been used since the early days of networking, back when computers were connected via telephone lines using modems, even before data networks were developed.

The most striking example of an overlay network is the Internet itself. The Internet itself was initially built as an overlay on the [[telephone network]].<ref name=over /> Even today, each Internet node can communicate with virtually any other through an underlying mesh of sub-networks of wildly different topologies and technologies. [[Address Resolution Protocol|Address resolution]] and [[routing]] are the means that allow mapping of a fully connected IP overlay network to its underlying network.

Another example of an overlay network is a [[distributed hash table]], which maps keys to nodes in the network. In this case, the underlying network is an IP network, and the overlay network is a table (actually a [[associative array|map]]) indexed by keys.

Overlay networks have also been proposed as a way to improve Internet routing, such as through [[quality of service]] guarantees achieve higher-quality [[streaming media]]. Previous proposals such as [[IntServ]], [[DiffServ]], and [[IP multicast]] have not seen wide acceptance largely because they require modification of all [[Router (computing)|routers]] in the network.{{Citation needed|date=August 2010}} On the other hand, an overlay network can be incrementally deployed on end-hosts running the overlay protocol software, without cooperation from [[Internet service provider]]s. The overlay network has no control over how packets are routed in the underlying network between two overlay nodes, but it can control, for example, the sequence of overlay nodes that a message traverses before it reaches its destination{{Citation needed|date=January 2023}}.

For example, [[Akamai Technologies]] manages an overlay network that provides reliable, efficient content delivery (a kind of [[multicast]]). Academic research includes end system multicast,<ref>{{cite web |title= End System Multicast |work= project web site |url= https://esm.cs.cmu.edu/ |publisher= Carnegie Mellon University |access-date=2013-05-25 |url-status= dead |archive-url= https://web.archive.org/web/20050221110350/https://esm.cs.cmu.edu/ |archive-date=2005-02-21 }}</ref> resilient routing and quality of service studies, among others.

== Network links ==
{{further|Data transmission}}
The transmission media (often referred to in the literature as the ''[[Transmission medium|physical medium]]'') used to link devices to form a computer network include [[electrical cable]], [[optical fiber]], and free space. In the [[OSI model]], the software to handle the media is defined at layers 1 and 2 — the physical layer and the data link layer.

A widely adopted ''family'' that uses copper and fiber media in [[local area network]] (LAN) technology are collectively known as Ethernet. The media and protocol standards that enable communication between networked devices over Ethernet are defined by [[IEEE 802.3]]. Wireless LAN standards use [[radio waves]], others use [[IrDA|infrared]] signals as a transmission medium. [[Power line communication]] uses a building's [[power cabling]] to transmit data.

=== Wired ===
[[File:Fibreoptic.jpg|thumb|upright=0.7|alt=Bundle of glass threads with light emitting from the ends|[[Fiber-optic cable]]s are used to transmit light from one computer/network node to another.]]
The following classes of wired technologies are used in computer networking.
* ''[[Coaxial cable]]'' is widely used for cable television systems, office buildings, and other work-sites for local area networks. Transmission speed ranges from 200 million bits per second to more than 500 million bits per second.{{citation needed|reason=Coax is fundamentally capable of more than this. This must refer to a specific [[Ethernet over coax]] standard, but that's covered in the point below.|date=September 2018}}
* [[ITU-T]] [[G.hn]] technology uses existing [[home wiring]] ([[Ethernet over coax|coaxial cable]], phone lines and [[Power line communication|power lines]]) to create a high-speed local area network.
* ''[[Twisted pair]]'' cabling is used for wired [[Ethernet]] and other standards. It typically consists of 4 pairs of copper cabling that can be utilized for both voice and data transmission. The use of two wires twisted together helps to reduce [[crosstalk]] and [[electromagnetic induction]]. The transmission speed ranges from 2&nbsp;Mbit/s to 10&nbsp;Gbit/s. Twisted pair cabling comes in two forms: unshielded twisted pair (UTP) and shielded twisted-pair (STP). Each form comes in several [[category cable|category ratings]], designed for use in various scenarios.

[[File:World map of submarine cables.png|thumb|alt=World map with red and blue lines|2007 map showing submarine optical fiber telecommunication cables around the world]]
* An ''[[optical fiber]]'' is a glass fiber. It carries pulses of light that represent data via lasers and [[optical amplifier]]s. Some advantages of optical fibers over metal wires are very low transmission loss and immunity to electrical interference. Using dense [[wave division multiplexing]], optical fibers can simultaneously carry multiple streams of data on different wavelengths of light, which greatly increases the rate that data can be sent to up to trillions of bits per second. Optic fibers can be used for long runs of cable carrying very high data rates, and are used for [[undersea communications cables]] to interconnect continents. There are two basic types of fiber optics, [[single-mode optical fiber]] (SMF) and [[multi-mode optical fiber]] (MMF). Single-mode fiber has the advantage of being able to sustain a coherent signal for dozens or even a hundred kilometers. Multimode fiber is cheaper to terminate but is limited to a few hundred or even only a few dozens of meters, depending on the data rate and cable grade.<ref name="1961- 2012">{{Cite book |title=CompTIA Network+ exam guide : (Exam N10-005) |last=Meyers |first=Mike |date=2012 |publisher=McGraw-Hill |isbn=9780071789226 |edition=5th |location=New York |oclc=748332969}}</ref>

=== Wireless ===
[[File: Wireless network.jpg|thumb|right|alt=Black laptop with the router in the background |Computers are very often connected to networks using wireless links.]]
{{Main|Wireless network}}
Network connections can be established wirelessly using radio or other electromagnetic means of communication.
* '' Terrestrial [[microwave]]''&nbsp;– Terrestrial microwave communication uses Earth-based transmitters and receivers resembling satellite dishes. Terrestrial microwaves are in the low gigahertz range, which limits all communications to line-of-sight. Relay stations are spaced approximately {{convert|40|mi|km}} apart.
* ''[[Communications satellite]]s''&nbsp;– Satellites also communicate via microwave. The satellites are stationed in space, typically in geosynchronous orbit {{convert|35,400|km|mi|abbr=on}} above the equator. These Earth-orbiting systems are capable of receiving and relaying voice, data, and TV signals.
* ''[[Cellular network]]s'' use several radio communications technologies. The systems divide the region covered into multiple geographic areas. Each area is served by a low-power [[transceiver]].
* ''Radio and [[spread spectrum]] technologies''&nbsp;– Wireless LANs use a high-frequency radio technology similar to digital cellular. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. [[IEEE 802.11]] defines a common flavor of open-standards wireless radio-wave technology known as [[Wi-Fi]].
* ''[[Free-space optical communication]]'' uses visible or invisible light for communications. In most cases, [[line-of-sight propagation]] is used, which limits the physical positioning of communicating devices.
* Extending the Internet to interplanetary dimensions via radio waves and optical means, the [[Interplanetary Internet]].<ref>{{citation |url=https://www.ipnsig.org/reports/ISART9-2000.pdf |archive-url=https://web.archive.org/web/20120113053223/https://www.ipnsig.org/reports/ISART9-2000.pdf |archive-date=2012-01-13 |url-status=dead |title=Interplanetary Internet |publisher=Third Annual International Symposium on Advanced Radio Technologies |author=A. Hooke |date=September 2000 |access-date=2011-11-12}}</ref>
* [[IP over Avian Carriers]] was a humorous April fool's [[Request for Comments]], issued as {{IETF RFC|1149}}. It was implemented in real life in 2001.<ref>{{cite web |url=https://www.blug.linux.no/rfc1149 |title=Bergen Linux User Group's CPIP Implementation |publisher=Blug.linux.no |access-date=2014-03-01 |archive-date=2014-02-15 |archive-url=https://web.archive.org/web/20140215072548/http://www.blug.linux.no/rfc1149/ |url-status=live}}</ref>

The last two cases have a large [[round-trip delay time]], which gives slow [[two-way communication]] but does not prevent sending large amounts of information (they can have high throughput).

== Network nodes ==
{{Main|Node (networking)}}
Apart from any physical transmission media, networks are built from additional basic system building blocks, such as [[network interface controller]]s, [[repeater]]s, [[Ethernet hub|hubs]], [[Network bridge|bridges]], [[Network switch|switches]], [[Router (computing)|routers]], modems, and [[Firewall (computing)|firewalls]]. Any particular piece of equipment will frequently contain multiple building blocks and so may perform multiple functions.

=== Network interfaces ===
{{main|Network interface controller}}
[[File: ForeRunnerLE 25 ATM Network Interface (1).jpg|thumb|right|alt=A network interface circuit with a port for ATM|An [[Asynchronous Transfer Mode|ATM]] network interface in the form of an accessory card. A lot of network interfaces are built-in.]]

A network interface controller (NIC) is [[computer hardware]] that connects the computer to the [[network media]] and has the ability to process low-level network information. For example, the NIC may have a connector for plugging in a cable, or an aerial for wireless transmission and reception, and the associated circuitry.

In Ethernet networks, each NIC has a unique [[MAC address|Media Access Control (MAC) address]]—usually stored in the controller's permanent memory. To avoid address conflicts between network devices, the [[Institute of Electrical and Electronics Engineers]] (IEEE) maintains and administers MAC address uniqueness. The size of an Ethernet MAC address is six [[Octet (computing)|octets]]. The three most significant octets are reserved to identify NIC manufacturers. These manufacturers, using only their assigned prefixes, uniquely assign the three least-significant octets of every Ethernet interface they produce.

=== Repeaters and hubs ===
{{main|Repeater}}

A repeater is an electronic device that receives a network [[signal]], cleans it of unnecessary noise and regenerates it. The signal is [[retransmission (data networks)|retransmitted]] at a higher power level, or to the other side of obstruction so that the signal can cover longer distances without degradation. In most twisted-pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. With fiber optics, repeaters can be tens or even hundreds of kilometers apart.

Repeaters work on the physical layer of the OSI model but still require a small amount of time to regenerate the signal. This can cause a [[propagation delay]] that affects network performance and may affect proper function. As a result, many network architectures limit the number of repeaters used in a network, e.g., the Ethernet [[5-4-3 rule]].

An Ethernet repeater with multiple ports is known as an [[Ethernet hub]]. In addition to reconditioning and distributing network signals, a repeater hub assists with collision detection and fault isolation for the network. Hubs and repeaters in LANs have been largely obsoleted by modern network switches.

===Bridges and switches===
{{main|Network bridge|Network switch}}

Network bridges and network switches are distinct from a hub in that they only forward frames to the ports involved in the communication whereas a hub forwards to all ports.<ref name=compnetworking>{{cite web |author=Bradley Mitchell |url=https://compnetworking.about.com/cs/internetworking/g/bldef_bridge.htm |title=bridge – network bridges |website=About.com |url-status=dead |archive-url=https://web.archive.org/web/20080328214940/https://compnetworking.about.com/cs/internetworking/g/bldef_bridge.htm |archive-date=2008-03-28}}</ref> Bridges only have two ports but a switch can be thought of as a multi-port bridge. Switches normally have numerous ports, facilitating a star topology for devices, and for cascading additional switches.

Bridges and switches operate at the [[data link layer]] (layer 2) of the OSI model and bridge traffic between two or more [[network segment]]s to form a single local network. Both are devices that forward [[Frame (networking)|frames]] of data between [[Computer port (hardware)|ports]] based on the destination MAC address in each frame.<ref>{{cite web
|url=https://www.webopedia.com/TERM/s/switch.html
|title=Define switch.
|website=webopedia
|date=September 1996
|access-date=2008-04-08
|archive-date=2008-04-08
|archive-url=https://web.archive.org/web/20080408102559/http://www.webopedia.com/TERM/S/switch.html
|url-status=live
}}</ref>
They learn the association of physical ports to MAC addresses by examining the source addresses of received frames and only forward the frame when necessary. If an unknown destination MAC is targeted, the device broadcasts the request to all ports except the source, and discovers the location from the reply.

Bridges and switches divide the network's collision domain but maintain a single broadcast domain. Network segmentation through bridging and switching helps break down a large, congested network into an aggregation of smaller, more efficient networks.

=== Routers ===
{{main|Router (computing)}}

[[File:Adsl connections.jpg|thumb|right|A typical home or small office router showing the [[ADSL]] telephone line and [[Ethernet]] network cable connections]]

A router is an internetworking device that forwards packets between networks by processing the addressing or routing information included in the packet. The routing information is often processed in conjunction with the [[routing table]]. A router uses its routing table to determine where to forward packets and does not require broadcasting packets which is inefficient for very big networks.

=== Modems ===
{{main|Modem}}

Modems (modulator-demodulator) are used to connect network nodes via wire not originally designed for digital network traffic, or for wireless. To do this one or more [[carrier signal]]s are [[modulated]] by the digital signal to produce an [[analog signal]] that can be tailored to give the required properties for transmission. Early modems modulated [[audio signal]]s sent over a standard voice telephone line. Modems are still commonly used for telephone lines, using a [[digital subscriber line]] technology and cable television systems using [[DOCSIS]] technology.

=== Firewalls ===
{{main|Firewall (computing)}}
[[File:Gateway firewall.svg|thumb|This is an image of a firewall separating a private network from a public network]]
A firewall is a network device or software for controlling network security and access rules. Firewalls are inserted in connections between secure internal networks and potentially insecure external networks such as the Internet. Firewalls are typically configured to reject access requests from unrecognized sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in [[cyber attack]]s.

== Communication protocols ==
[[File:Internet layering.svg|250px|thumb|alt=Protocols in relation to the Internet layering scheme.|The TCP/IP model and its relation to common protocols used at different layers of the model]]
[[File:Message flows and Routing.svg|250px|thumb|alt=When a router is present, message flows go down through protocol layers, across to the router, up the stack inside the router, and back down again and is sent on to the final destination where it climbs back up the stack|Message flows between two devices (A-B) at the four layers of the TCP/IP model in the presence of a router (R). Red flows are effective communication paths, black paths are across the actual network links.]]

A [[communication protocol]] is a set of rules for exchanging information over a network. Communication protocols have various characteristics. They may be [[connection-oriented]] or [[connectionless]], they may use [[circuit mode]] or packet switching, and they may use hierarchical addressing or flat addressing.

In a [[protocol stack]], often constructed per the OSI model, communications functions are divided up into protocol layers, where each layer leverages the services of the layer below it until the lowest layer controls the hardware that sends information across the media. The use of protocol layering is ubiquitous across the field of computer networking. An important example of a protocol stack is [[HTTP]] (the World Wide Web protocol) running over [[transmission control protocol|TCP]] over IP (the Internet protocols) over [[IEEE 802.11]] (the Wi-Fi protocol). This stack is used between the [[wireless router]] and the home user's personal computer when the user is surfing the web.

There are many communication protocols, a few of which are described below.
{{Further|Rate Based Satellite Control Protocol}}

===Common protocols===
==== Internet protocol suite ====
The [[Internet protocol suite]], also called TCP/IP, is the foundation of all modern networking. It offers connection-less and connection-oriented services over an inherently unreliable network traversed by datagram transmission using Internet protocol (IP). At its core, the protocol suite defines the addressing, identification, and routing specifications for [[Internet Protocol Version 4]] (IPv4) and for [[IPv6]], the next generation of the protocol with a much enlarged addressing capability. The Internet protocol suite is the defining set of protocols for the Internet.<ref name=Tanenbaum>{{cite book|first=Andrew S.|last=Tanenbaum|author-link=Andrew S. Tanenbaum|title=Computer Networks|edition=4th|publisher=[[Prentice Hall]]|date=2003}}</ref>

==== IEEE 802 ====
[[IEEE 802]] is a family of IEEE standards dealing with local area networks and metropolitan area networks. The complete IEEE 802 protocol suite provides a diverse set of networking capabilities. The protocols have a flat addressing scheme. They operate mostly at layers 1 and 2 of the OSI model.

For example, [[Bridging (networking)|MAC bridging]] ([[IEEE 802.1D]]) deals with the routing of Ethernet packets using a [[Spanning Tree Protocol]]. [[IEEE 802.1Q]] describes [[VLAN]]s, and [[IEEE 802.1X]] defines a port-based [[network access control]] protocol, which forms the basis for the authentication mechanisms used in VLANs<ref>{{cite journal |url=https://ieeexplore.ieee.org/document/9018454 |date=February 2020 |doi=10.1109/IEEESTD.2020.9018454 |isbn=978-1-5044-6440-6 |at=7.1.3 Connectivity to unauthenticated systems |access-date=2022-05-09 |archive-date=2023-02-04 |archive-url=https://web.archive.org/web/20230204143515/https://ieeexplore.ieee.org/document/9018454 |url-status=live |title=IEEE Standard for Local and Metropolitan Area Networks--Port-Based Network Access Control |journal=IEEE STD 802.1X-2020 (Revision of IEEE STD 802.1X-2010 Incorporating IEEE STD 802.1Xbx-2014 and IEEE STD 802.1Xck-2018) |url-access=subscription }}</ref> (but it is also found in WLANs<ref>{{cite journal |url=https://ieeexplore.ieee.org/document/9363693 |date=February 2021 |doi=10.1109/IEEESTD.2021.9363693 |isbn=978-1-5044-7283-8 |at=4.2.5 Interaction with other IEEE 802 layers |access-date=2022-05-09 |archive-date=2022-05-17 |archive-url=https://web.archive.org/web/20220517014920/https://ieeexplore.ieee.org/document/9363693 |url-status=live |title=IEEE Standard for Information Technology--Telecommunications and Information Exchange between Systems - Local and Metropolitan Area Networks--Specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications |journal=IEEE STD 802.11-2020 (Revision of IEEE STD 802.11-2016) |url-access=subscription }}</ref>)&nbsp;– it is what the home user sees when the user has to enter a "wireless access key".

===== Ethernet =====
Ethernet is a family of technologies used in wired LANs. It is described by a set of standards together called [[IEEE 802.3]] published by the Institute of Electrical and Electronics Engineers.

===== Wireless LAN =====
Wireless LAN based on the [[IEEE 802.11]] standards, also widely known as WLAN or WiFi, is probably the most well-known member of the [[IEEE 802]] protocol family for home users today. IEEE 802.11 shares many properties with wired Ethernet.

==== SONET/SDH ====
[[Synchronous optical networking]] (SONET) and Synchronous Digital Hierarchy (SDH) are standardized [[multiplexing]] protocols that transfer multiple digital bit streams over optical fiber using lasers. They were originally designed to transport circuit mode communications from a variety of different sources, primarily to support circuit-switched [[digital telephony]]. However, due to its protocol neutrality and transport-oriented features, SONET/SDH also was the obvious choice for transporting [[Asynchronous Transfer Mode]] (ATM) frames.

==== Asynchronous Transfer Mode ====
Asynchronous Transfer Mode (ATM) is a switching technique for telecommunication networks. It uses asynchronous [[time-division multiplexing]] and encodes data into small, fixed-sized [[cell relay|cells]]. This differs from other protocols such as the Internet protocol suite or [[Ethernet]] that use variable-sized packets or [[Ethernet frame|frames]]. ATM has similarities with both circuit and packet switched networking. This makes it a good choice for a network that must handle both traditional high-throughput data traffic, and real-time, [[Network latency|low-latency]] content such as voice and video. ATM uses a connection-oriented model in which a [[virtual circuit]] must be established between two endpoints before the actual data exchange begins.

ATM still plays a role in the [[Last mile (telecommunications)|last mile]], which is the connection between an Internet service provider and the home user.<ref>{{cite web|last=Martin|first=Thomas|title=Design Principles for DSL-Based Access Solutions|url=https://www.gsi.dit.upm.es/~legf/Varios/XDSL_MARTI.PDF|archive-url=https://web.archive.org/web/20110722194039/https://www.gsi.dit.upm.es/~legf/Varios/XDSL_MARTI.PDF|archive-date=2011-07-22|url-status=dead}}</ref>{{update inline|date=January 2022}}

==== Cellular standards ====
There are a number of different digital cellular standards, including: [[Global System for Mobile Communications]] (GSM), [[General Packet Radio Service]] (GPRS), [[cdmaOne]], [[CDMA2000]], [[Evolution-Data Optimized]] (EV-DO), [[Enhanced Data Rates for GSM Evolution]] (EDGE), [[Universal Mobile Telecommunications System]] (UMTS), [[Digital Enhanced Cordless Telecommunications]] (DECT), [[Digital AMPS]] (IS-136/TDMA), and [[Integrated Digital Enhanced Network]] (iDEN).<ref>{{cite book |last=Paetsch |first=Michael |year=1993 |title=The evolution of mobile communications in the US and Europe: Regulation, technology, and markets |location=Boston, London |publisher=Artech House |isbn=978-0-8900-6688-1}}</ref>

=== Routing ===
[[File:XO classroom network.jpg|thumbnail|Routing calculates good paths through a network for information to take. For example, from node 1 to node 6 the best routes are likely to be 1-8-7-6, 1-8-10-6 or 1-9-10-6, as these are the shortest routes.]]

[[Routing]] is the process of selecting network paths to carry network traffic. Routing is performed for many kinds of networks, including circuit switching networks and packet switched networks.

In packet-switched networks, [[routing protocol]]s direct [[packet forwarding]] through intermediate nodes. Intermediate nodes are typically network hardware devices such as routers, bridges, gateways, firewalls, or switches. General-purpose [[computer]]s can also forward packets and perform routing, though because they lack specialized hardware, may offer limited performance. The routing process directs forwarding on the basis of [[routing table]]s, which maintain a record of the routes to various network destinations. Most routing algorithms use only one network path at a time. [[Multipath routing]] techniques enable the use of multiple alternative paths.

Routing can be contrasted with [[bridging (networking)|bridging]] in its assumption that [[network address]]es are structured and that similar addresses imply proximity within the network. Structured addresses allow a single routing table entry to represent the route to a group of devices. In large networks, the structured addressing used by routers outperforms unstructured addressing used by bridging. Structured IP addresses are used on the Internet. Unstructured MAC addresses are used for bridging on Ethernet and similar local area networks.

== Geographic scale ==
{{Area networks}}
Networks may be characterized by many properties or features, such as physical capacity, organizational purpose, user authorization, access rights, and others. Another distinct classification method is that of the physical extent or geographic scale.

===Nanoscale network===
A [[nanoscale network]] has key components implemented at the nanoscale, including message carriers, and leverages physical principles that differ from macroscale communication mechanisms. Nanoscale communication extends communication to very small sensors and actuators such as those found in biological systems and also tends to operate in environments that would be too harsh for other communication techniques.<ref>{{cite book |title=Nanoscale Communication Networks |last=Bush |first=S. F. |isbn=978-1-60807-003-9 |publisher=Artech House |year=2010 }}</ref>

===Personal area network===
A [[personal area network]] (PAN) is a computer network used for communication among computers and different information technological devices close to one person. Some examples of devices that are used in a PAN are personal computers, printers, fax machines, telephones, PDAs, scanners, and video game consoles. A PAN may include wired and wireless devices. The reach of a PAN typically extends to 10 meters.<ref>{{cite web |url=https://searchmobilecomputing.techtarget.com/sDefinition/0,,sid40_gci546288,00.html |title=personal area network (PAN) |author=Margaret Rouse |website=TechTarget |access-date=2011-01-29 |archive-date=2023-02-04 |archive-url=https://web.archive.org/web/20230204143532/https://www.techtarget.com/searchmobilecomputing/definition/personal-area-network |url-status=live }}</ref> A wired PAN is usually constructed with [[USB]] and [[FireWire]] connections while technologies such as [[Bluetooth]] and [[infrared communication]] typically form a wireless PAN.

===Local area network===
A [[local area network]] (LAN) is a network that connects computers and devices in a limited geographical area such as a home, school, office building, or closely positioned group of buildings. Wired LANs are most commonly based on Ethernet technology. Other networking technologies such as [[ITU-T]] [[G.hn]] also provide a way to create a wired LAN using existing wiring, such as coaxial cables, telephone lines, and power lines.<ref>{{cite web |url=https://www.itu.int/ITU-T/newslog/New+Global+Standard+For+Fully+Networked+Home.aspx |title=New global standard for fully networked home |publisher=[[ITU]] |website=ITU-T Newslog |access-date=2011-11-12 |date=2008-12-12 |url-status=dead |archive-url=https://web.archive.org/web/20090221090736/https://www.itu.int/ITU-T/newslog/New+Global+Standard+For+Fully+Networked+Home.aspx |archive-date=2009-02-21 }}</ref>

A LAN can be connected to a [[wide area network]] (WAN) using a router. The defining characteristics of a LAN, in contrast to a WAN, include higher [[data transfer rate]]s, limited geographic range, and lack of reliance on [[leased line]]s to provide connectivity.{{citation needed|reason=IEEE ref below is narrowly focused on 100G ethernet|date=August 2022}} Current Ethernet or other IEEE 802.3 LAN technologies operate at data transfer rates up to and in excess of [[100 Gigabit Ethernet|100&nbsp;Gbit/s]],<ref>{{cite web |website=IEEE 802.3 ETHERNET WORKING GROUP |url=https://www.ieee802.org/3/ba/ |title=IEEE P802.3ba 40Gb/s and 100Gb/s Ethernet Task Force |access-date=2011-11-12 |archive-date=2011-11-20 |archive-url=https://web.archive.org/web/20111120020900/http://www.ieee802.org/3/ba/ |url-status=live }}</ref> standardized by IEEE in 2010.

===Home area network===
A [[home area network]] (HAN) is a residential LAN used for communication between digital devices typically deployed in the home, usually a small number of personal computers and accessories, such as printers and mobile computing devices. An important function is the sharing of Internet access, often a broadband service through a [[cable Internet access]] or [[digital subscriber line]] (DSL) provider.

===Storage area network===
A [[storage area network]] (SAN) is a dedicated network that provides access to consolidated, block-level data storage. SANs are primarily used to make storage devices, such as disk arrays, tape libraries, and optical jukeboxes, accessible to servers so that the storage appears as locally attached devices to the operating system. A SAN typically has its own network of storage devices that are generally not accessible through the local area network by other devices. The cost and complexity of SANs dropped in the early 2000s to levels allowing wider adoption across both enterprise and small to medium-sized business environments.{{citation needed|date=September 2022}}

===Campus area network===
A [[campus area network]] (CAN) is made up of an interconnection of LANs within a limited geographical area. The networking equipment (switches, routers) and transmission media (optical fiber, [[Category 5 cable|Cat5]] cabling, etc.) are almost entirely owned by the campus tenant or owner (an enterprise, university, government, etc.).

For example, a university campus network is likely to link a variety of campus buildings to connect academic colleges or departments, the library, and student residence halls.

===Backbone network===
A [[backbone network]] is part of a computer network infrastructure that provides a path for the exchange of information between different LANs or subnetworks. A backbone can tie together diverse networks within the same building, across different buildings, or over a wide area. When designing a network backbone, [[Network performance management|network performance]] and [[network congestion]] are critical factors to take into account. Normally, the backbone network's capacity is greater than that of the individual networks connected to it.

For example, a large company might implement a backbone network to connect departments that are located around the world. The equipment that ties together the departmental networks constitutes the network backbone. Another example of a backbone network is the [[Internet backbone]], which is a massive, global system of fiber-optic cable and optical networking that carry the bulk of data between [[wide area network]]s (WANs), metro, regional, national and transoceanic networks.

===Metropolitan area network===
A [[metropolitan area network]] (MAN) is a large computer network that interconnects users with computer resources in a geographic region of the size of a [[metropolitan area]].

===Wide area network===
A [[wide area network]] (WAN) is a computer network that covers a large geographic area such as a city, country, or spans even intercontinental distances. A WAN uses a communications channel that combines many types of media such as telephone lines, cables, and airwaves. A WAN often makes use of transmission facilities provided by [[common carrier]]s, such as telephone companies. WAN technologies generally function at the lower three layers of the OSI model: the physical layer, the [[data link layer]], and the [[network layer]].

===Enterprise private network===
An [[enterprise private network]] is a network that a single organization builds to interconnect its office locations (e.g., production sites, head offices, remote offices, shops) so they can share computer resources.

===Virtual private network===
A [[virtual private network]] (VPN) is an [[overlay network]] in which some of the links between nodes are carried by open connections or virtual circuits in some larger network (e.g., the Internet) instead of by physical wires. The data link layer protocols of the virtual network are said to be tunneled through the larger network. One common application is secure communications through the public Internet, but a VPN need not have explicit security features, such as authentication or content encryption. VPNs, for example, can be used to separate the traffic of different user communities over an underlying network with strong security features.

VPN may have best-effort performance or may have a defined service level agreement (SLA) between the VPN customer and the VPN service provider.

===Global area network===
A [[global area network]] (GAN) is a network used for supporting mobile users across an arbitrary number of wireless LANs, satellite coverage areas, etc. The key challenge in mobile communications is handing off communications from one local coverage area to the next. In IEEE Project 802, this involves a succession of terrestrial [[wireless LAN]]s.<ref>{{cite web |url=https://grouper.ieee.org/groups/802/20/ |website=IEEE 802.20 — Mobile Broadband Wireless Access (MBWA) |title=IEEE 802.20 Mission and Project Scope |access-date=2011-11-12}}</ref>

== Organizational scope ==
Networks are typically managed by the organizations that own them. Private enterprise networks may use a combination of intranets and extranets. They may also provide network access to the Internet, which has no single owner and permits virtually unlimited global connectivity.

=== Intranet ===
An [[intranet]] is a set of networks that are under the control of a single administrative entity. An intranet typically uses the Internet Protocol and IP-based tools such as web browsers and file transfer applications. The administrative entity limits the use of the intranet to its authorized users. Most commonly, an intranet is the internal LAN of an organization. A large intranet typically has at least one web server to provide users with organizational information.

=== Extranet ===
An [[extranet]] is a network that is under the administrative control of a single organization but supports a limited connection to a specific external network. For example, an organization may provide access to some aspects of its intranet to share data with its business partners or customers. These other entities are not necessarily trusted from a security standpoint. The network connection to an extranet is often, but not always, implemented via WAN technology.

=== Internet ===
[[File:Internet map 1024.jpg|thumb|right|Partial map of the Internet based on 2005 data.<ref>{{cite web|url=https://www.opte.org/maps/ |title=Maps |website=The Opto Project |archive-url=https://web.archive.org/web/20050115092114/https://www.opte.org/maps/ |archive-date=2005-01-15 |url-status=dead}}</ref> Each line is drawn between two nodes, representing two [[IP address]]es. The length of the lines indicates the delay between those two nodes.]]

An [[internetwork]] is the connection of multiple different types of computer networks to form a single computer network using higher-layer network protocols and connecting them together using routers.

The [[Internet]] is the largest example of internetwork. It is a global system of interconnected governmental, academic, corporate, public, and private computer networks. It is based on the networking technologies of the Internet protocol suite. It is the successor of the [[Advanced Research Projects Agency Network]] (ARPANET) developed by [[DARPA]] of the [[United States Department of Defense]]. The Internet utilizes copper communications and an [[optical networking]] backbone to enable the [[World Wide Web]] (WWW), the [[Internet of things]], video transfer, and a broad range of information services.

Participants on the Internet use a diverse array of methods of several hundred documented, and often standardized, protocols compatible with the Internet protocol suite and the IP addressing system administered by the [[Internet Assigned Numbers Authority]] and [[regional Internet registry|address registries]]. Service providers and large enterprises exchange information about the reachability of their address spaces through the [[Border Gateway Protocol]] (BGP), forming a redundant worldwide mesh of transmission paths.

=== Darknet ===
A [[darknet]] is an overlay network, typically running on the Internet, that is only accessible through specialized software. It is an anonymizing network where connections are made only between trusted peers&nbsp;— sometimes called ''friends'' ([[Friend-to-friend|F2F]])<ref>{{Cite journal
| last1 = Mansfield-Devine | first1 = Steve
| title = Darknets
| doi = 10.1016/S1361-3723(09)70150-2
| journal = Computer Fraud & Security
| volume = 2009
| issue = 12
| pages = 4–6
|date=December 2009
}}</ref>&nbsp;— using non-standard protocols and [[Port (computer networking)|ports]].

Darknets are distinct from other distributed [[peer-to-peer]] networks as [[Peer-to-peer file sharing|sharing]] is anonymous (that is, IP addresses are not publicly shared), and therefore users can communicate with little fear of governmental or corporate interference.<ref name=Wood>{{cite journal |last=Wood |first=Jessica |title=The Darknet: A Digital Copyright Revolution |journal=Richmond Journal of Law and Technology |year=2010 |volume=16 |issue=4 |url=https://jolt.richmond.edu/v16i4/article14.pdf |access-date=2011-10-25 |archive-date=2012-04-15 |archive-url=https://web.archive.org/web/20120415112930/http://jolt.richmond.edu/v16i4/article14.pdf |url-status=live }}</ref>

== Network service ==
[[Network service]]s are applications hosted by servers on a computer network, to [[Service (systems architecture)|provide some functionality]] for members or users of the network, or to help the network itself to operate.

The [[World Wide Web]], [[E-mail]],<ref>{{cite ietf|title=Simple Mail Transfer Protocol|rfc=5321|first=J.|last=Klensin|date=October 2008}}</ref> [[printing]] and [[distributed file system|network file sharing]] are examples of well-known network services. Network services such as [[Domain Name System]] (DNS) give names for [[IP address|IP]] and [[MAC address]]es (people remember names like ''nm.lan'' better than numbers like {{IPaddr|210.121.67.18}}),<ref name="rfc1035">{{cite ietf|rfc=1035|title=Domain names – Implementation and Specification|first=P.|last=Mockapetris|date= November 1987}}</ref> and [[Dynamic Host Configuration Protocol]] (DHCP) to ensure that the equipment on the network has a valid IP address.<ref>{{cite book |last1=Peterson |first1=L.L. |last2=Davie |first2=B.S. |year=2011 |url=https://books.google.com/books?id=BvaFreun1W8C&pg=PA372 |title=Computer Networks: A Systems Approach |page=372 |edition=5th |publisher=Elsevier |isbn=978-0-1238-5060-7}}</ref>

Services are usually based on a [[protocol (computing)|service protocol]] that defines the format and sequencing of messages between clients and servers of that network service.

== Network performance ==
=== Bandwidth ===
[[Bandwidth (computing)|Bandwidth]] in [[bit/s]] may refer to consumed bandwidth, corresponding to achieved [[throughput]] or [[goodput]], i.e., the average rate of successful data transfer through a communication path. The throughput is affected by processes such as [[bandwidth shaping]], [[bandwidth management]], [[bandwidth throttling]], [[bandwidth cap]] and [[bandwidth allocation]] (using, for example, [[bandwidth allocation protocol]] and [[dynamic bandwidth allocation]]).

===Network delay===
{{main|Network delay}}
''Network delay'' is a design and performance characteristic of a [[telecommunications network]]. It specifies the [[Network latency|latency]] for a bit of data to travel across the network from one [[communication endpoint]] to another. Delay may differ slightly, depending on the location of the specific pair of communicating endpoints. Engineers usually report both the maximum and average delay, and they divide the delay into several components, the sum of which is the total delay:
* [[Processing delay]]{{snd}} time it takes a router to process the packet header
* [[Queuing delay]]{{snd}} time the packet spends in routing queues
* [[Transmission delay]]{{snd}} time it takes to push the packet's bits onto the link
* [[Propagation delay]]{{snd}} time for a signal to propagate through the media

A certain minimum level of delay is experienced by signals due to the time it takes to [[Data transmission|transmit]] a packet serially through a [[Data link|link]]. This delay is extended by more variable levels of delay due to [[network congestion]]. [[IP network]] delays can range from less than a microsecond to several hundred milliseconds.

=== Performance metrics ===
The parameters that affect performance typically can include [[throughput]], [[jitter]], [[bit error rate]] and latency.

In circuit-switched networks, network performance is synonymous with the [[grade of service]]. The number of rejected calls is a measure of how well the network is performing under heavy traffic loads.<ref>{{cite book |url=https://www.com.dtu.dk/teletraffic/handbook/telenook.pdf |url-status=dead |archive-url=https://web.archive.org/web/20070111015452/https://oldwww.com.dtu.dk/teletraffic/handbook/telenook.pdf |archive-date=2007-01-11 |author=ITU-D Study Group 2 |date=June 2006 |title=Teletraffic Engineering Handbook}}</ref> Other types of performance measures can include the level of noise and echo.

In an [[Asynchronous Transfer Mode]] (ATM) network, performance can be measured by line rate, quality of service (QoS), data throughput, connect time, stability, technology, modulation technique, and modem enhancements.<ref>{{cite web|url=https://www.telecommagazine.com/|url-status=dead|title=Telecommunications Magazine Online|archive-url=https://web.archive.org/web/20110208160737/http://telecommagazine.com/ |archive-date=2011-02-08|date=January 2003}}</ref>{{verify source |date=August 2018}}{{full citation needed|date=August 2018}}

There are many ways to measure the performance of a network, as each network is different in nature and design. Performance can also be modeled instead of measured. For example, [[state diagram|state transition diagrams]] are often used to model queuing performance in a circuit-switched network. The network planner uses these diagrams to analyze how the network performs in each state, ensuring that the network is optimally designed.<ref>{{cite web |url=https://cne.gmu.edu/modules/os_perf/std.t.html |title=State Transition Diagrams |access-date=2003-07-13 |url-status=dead |archive-url=https://web.archive.org/web/20031015010139/https://cne.gmu.edu/modules/os_perf/std.t.html |archive-date=2003-10-15 }}</ref>

=== Network congestion ===
[[Network congestion]] occurs when a link or node is subjected to a greater data load than it is rated for, resulting in a deterioration of its quality of service. When networks are congested and queues become too full, packets have to be discarded, and participants must rely on [[Retransmission (data networks)|retransmission]] to maintain [[reliable communications]]. Typical effects of congestion include [[queueing delay]], [[packet loss]] or the [[blocking probability|blocking]] of new connections. A consequence of these latter two is that incremental increases in [[offered load]] lead either to only a small increase in the network [[throughput]] or to a potential reduction in network throughput.

[[Network protocol]]s that use aggressive retransmissions to compensate for packet loss tend to keep systems in a state of network congestion even after the initial load is reduced to a level that would not normally induce network congestion. Thus, networks using these protocols can exhibit two stable states under the same level of load. The stable state with low throughput is known as ''[[congestive collapse]]''.

Modern networks use [[congestion control]], [[congestion avoidance]] and [[Network traffic control|traffic control]] techniques where endpoints typically slow down or sometimes even stop transmission entirely when the network is congested to try to avoid congestive collapse. Specific techniques include: [[exponential backoff]] in protocols such as [[802.11]]'s [[CSMA/CA]] and the original Ethernet, [[sliding window|window]] reduction in TCP, and [[fair queueing]] in devices such as routers.

Another method to avoid the negative effects of network congestion is implementing [[quality of service]] priority schemes allowing selected traffic to bypass congestion. Priority schemes do not solve network congestion by themselves, but they help to alleviate the effects of congestion for critical services. A third method to avoid network congestion is the explicit allocation of network resources to specific flows. One example of this is the use of Contention-Free Transmission Opportunities (CFTXOPs) in the [[ITU-T]] [[G.hn]] home networking standard.

For the Internet, {{IETF RFC|2914}} addresses the subject of congestion control in detail.

=== Network resilience ===
[[Network resilience]] is "the ability to provide and maintain an acceptable level of [[Service (systems architecture)|service]] in the face of [[Fault (technology)|faults]] and challenges to normal operation."<ref>{{cite web |url=https://resilinets.org/definitions.html#Resilience |publisher=ResiliNets Research Initiative |title=Definitions: Resilience |access-date=2011-11-12 |archive-date=2020-11-06 |archive-url=https://web.archive.org/web/20201106133722/https://resilinets.org/definitions.html#Resilience |url-status=live }}</ref>

== Security ==
Computer networks are also used by [[security hacker]]s to deploy [[computer virus]]es or [[computer worm]]s on devices connected to the network, or to prevent these devices from accessing the network via a [[denial-of-service attack]].

=== Network security ===
[[Network Security]] consists of provisions and policies adopted by the [[network administrator]] to prevent and monitor [[unauthorized]] access, misuse, modification, or denial of the computer network and its network-accessible resources.<ref>{{cite book | doi = 10.1007/978-3-540-30176-9_41 | last = Simmonds | first = A |author2=Sandilands, P |author3=van Ekert, L| title = Applied Computing | chapter = An Ontology for Network Security Attacks | s2cid = 2204780 | volume = 3285 | pages = 317–323 | year = 2004 | series = Lecture Notes in Computer Science | isbn = 978-3-540-23659-7 }}</ref> Network security is used on a variety of computer networks, both public and private, to secure daily transactions and communications among businesses, government agencies, and individuals.

=== Network surveillance ===
[[Network surveillance]] is the monitoring of data being transferred over computer networks such as the Internet. The monitoring is often done surreptitiously and may be done by or at the behest of governments, by corporations, criminal organizations, or individuals. It may or may not be legal and may or may not require authorization from a court or other independent agency.

Computer and network surveillance programs are widespread today, and almost all Internet traffic is or could potentially be monitored for clues to illegal activity.

Surveillance is very useful to governments and [[law enforcement]] to maintain [[social control]], recognize and monitor threats, and prevent or investigate [[criminal]] activity. With the advent of programs such as the [[Total Information Awareness]] program, technologies such as high-speed surveillance computers and [[Surveillance#Biometric|biometrics]] software, and laws such as the [[Communications Assistance For Law Enforcement Act]], governments now possess an unprecedented ability to monitor the activities of citizens.<ref name="us-surveillance-soc">{{cite web |url=https://www.aclu.org/other/us-turning-surveillance-society |title=Is the U.S. Turning Into a Surveillance Society? |publisher=American Civil Liberties Union |access-date=2009-03-13 |archive-date=2017-03-14 |archive-url=https://web.archive.org/web/20170314094152/https://www.aclu.org/other/us-turning-surveillance-society |url-status=live }}</ref>

However, many [[civil rights]] and [[privacy]] groups—such as [[Reporters Without Borders]], the [[Electronic Frontier Foundation]], and the [[American Civil Liberties Union]]—have expressed concern that increasing surveillance of citizens may lead to a [[mass surveillance]] society, with limited political and personal freedoms. Fears such as this have led to lawsuits such as ''[[Hepting v. AT&T]]''.<ref name="us-surveillance-soc" /><ref name="bigger-monster">{{cite web |author1=Jay Stanley |author2=Barry Steinhardt |url=https://www.aclu.org/FilesPDFs/aclu_report_bigger_monster_weaker_chains.pdf |title=Bigger Monster, Weaker Chains: The Growth of an American Surveillance Society |date=January 2003 |publisher=American Civil Liberties Union |access-date=2009-03-13 |archive-date=2022-10-09 |archive-url=https://ghostarchive.org/archive/20221009/https://www.aclu.org/FilesPDFs/aclu_report_bigger_monster_weaker_chains.pdf |url-status=live }}</ref> The [[hacktivist]] group [[Anonymous (group)|Anonymous]] has hacked into government websites in protest of what it considers "draconian surveillance".<ref>{{cite web |url=https://www.zdnet.com/blog/security/anonymous-hacks-uk-government-sites-over-draconian-surveillance/11412 |title=Anonymous hacks UK government sites over 'draconian surveillance' |author=Emil Protalinski |website=ZDNet |date=2012-04-07 |access-date=12 March 2013 |archive-date=2013-04-03 |archive-url=https://web.archive.org/web/20130403054106/http://www.zdnet.com/blog/security/anonymous-hacks-uk-government-sites-over-draconian-surveillance/11412 |url-status=dead }}</ref><ref>{{cite web |url=https://www.theguardian.com/technology/2012/apr/20/hacktivists-battle-internet |title=Hacktivists in the frontline battle for the internet |author=James Ball |work=The Guardian |date=2012-04-20 |access-date=2012-06-17 |author-link=James Ball (journalist) |archive-date=2018-03-14 |archive-url=https://web.archive.org/web/20180314103619/https://www.theguardian.com/technology/2012/apr/20/hacktivists-battle-internet |url-status=live }}</ref>

=== End to end encryption ===
[[End-to-end encryption]] (E2EE) is a [[digital communications]] paradigm of uninterrupted protection of data traveling between two communicating parties. It involves the originating party [[encrypting]] data so only the intended recipient can decrypt it, with no dependency on third parties. End-to-end encryption prevents intermediaries, such as Internet service providers or [[application service provider]]s, from reading or tampering with communications. End-to-end encryption generally protects both [[confidentiality]] and [[data integrity|integrity]].

Examples of end-to-end encryption include [[HTTPS]] for web traffic, [[Pretty Good Privacy|PGP]] for [[email]], [[Off-the-Record Messaging|OTR]] for [[instant messaging]], [[ZRTP]] for [[telephony]], and [[TETRA]] for radio.

Typical [[Server (computing)|server]]-based communications systems do not include end-to-end encryption. These systems can only guarantee the protection of communications between [[Client (computing)|clients]] and [[Server (computing)|servers]], not between the communicating parties themselves. Examples of non-E2EE systems are [[Google Talk]], [[Yahoo Messenger]], [[Facebook]], and [[Dropbox]].

The end-to-end encryption paradigm does not directly address risks at the endpoints of the communication themselves, such as the [[Exploit (computer security)|technical exploitation]] of [[Client (computing)|clients]], poor quality [[random number generator]]s, or [[key escrow]]. E2EE also does not address [[traffic analysis]], which relates to things such as the identities of the endpoints and the times and quantities of messages that are sent.

=== SSL/TLS ===
{{Main|Transport Layer Security}}

The introduction and rapid growth of e-commerce on the World Wide Web in the mid-1990s made it obvious that some form of authentication and encryption was needed. [[Netscape]] took the first shot at a new standard. At the time, the dominant web browser was [[Netscape Navigator]]. Netscape created a standard called secure socket layer (SSL). SSL requires a server with a certificate. When a client requests access to an SSL-secured server, the server sends a copy of the certificate to the client. The SSL client checks this certificate (all web browsers come with an exhaustive list of [[root certificate]]s preloaded), and if the certificate checks out, the server is authenticated and the client negotiates a [[symmetric-key cipher]] for use in the session. The session is now in a very secure encrypted tunnel between the SSL server and the SSL client.<ref name="1961- 2012"/>

== Views of networks ==
Users and network administrators typically have different views of their networks. Users can share printers and some servers from a [[Workgroup (computer networking)|workgroup]], which usually means they are in the same geographic location and are on the same LAN, whereas a network administrator is responsible for keeping that network up and running. A [[community of interest]] has less of a connection of being in a local area and should be thought of as a set of arbitrarily located users who share a set of servers, and possibly also communicate via [[peer-to-peer]] technologies.

Network administrators can see networks from both physical and logical perspectives. The physical perspective involves geographic locations, physical cabling, and the network elements (e.g., routers, bridges and [[application-layer gateway]]s) that interconnect via the transmission media. Logical networks, called, in the TCP/IP architecture, [[subnet]]s, map onto one or more transmission media. For example, a common practice in a campus of buildings is to make a set of LAN cables in each building appear to be a common subnet, using VLANs.

Users and administrators are aware, to varying extents, of a network's trust and scope characteristics. Again using TCP/IP architectural terminology, an [[intranet]] is a community of interest under private administration usually by an enterprise, and is only accessible by authorized users (e.g. employees).<ref name="RFC2547">{{cite ietf|rfc=2547|title=BGP/MPLS VPNs|first1=E.|last1=Rosen|first2=Y.|last2=Rekhter|date=March 1999}}</ref> Intranets do not have to be connected to the Internet, but generally have a limited connection. An [[extranet]] is an extension of an intranet that allows secure communications to users outside of the intranet (e.g. business partners, customers).<ref name="RFC2547" />

Unofficially, the Internet is the set of users, enterprises, and content providers that are interconnected by Internet Service Providers (ISP). From an engineering viewpoint, the Internet is the set of subnets, and aggregates of subnets, that share the registered [[IP address]] space and exchange information about the reachability of those IP addresses using the [[Border Gateway Protocol]]. Typically, the [[human-readable]] names of servers are translated to IP addresses, transparently to users, via the directory function of the [[Domain Name System]] (DNS).

Over the Internet, there can be [[business-to-business]], [[business-to-consumer]] and [[consumer-to-consumer]] communications. When money or sensitive information is exchanged, the communications are apt to be protected by some form of [[communications security]] mechanism. Intranets and extranets can be securely superimposed onto the Internet, without any access by general Internet users and administrators, using secure VPN technology.

== See also ==
{{cmn|
* [[Cloud computing]]
* [[Cyberspace]]
* [[History of the Internet]]
* [[Information Age]]
* {{annotated link|ISO/IEC 11801}}
* [[Network diagram software]]
* [[Network mapping]]
* [[Network on a chip]]
* [[Network planning and design]]
* [[Network simulation]]
}}

== References ==
{{Reflist}}
{{FS1037C}}

== Further reading ==
* Kurose James F and Keith W. Ross: Computer Networking: A Top-Down Approach Featuring the Internet, Pearson Education 2005.
* [[William Stallings]], ''Computer Networking with Internet Protocols and Technology'', Pearson Education 2004.
* [[Dimitri Bertsekas]], and [[Robert Gallager]], "Data Networks," Prentice Hall, 1992.

{{Operating System}}
{{Computer science}}
{{Authority control}}

[[Category:Computer networks| ]]