View source for Autonomous system (Internet)

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{{short description|Internet routing system}}
An '''autonomous system''' ('''AS''') is a collection of connected [[Internet protocol address|Internet Protocol]] (IP) [[routing]] prefixes under the control of one or more network operators on behalf of a single administrative entity or domain, that presents a common and clearly defined routing policy to the Internet.<ref name=rfc1930>{{cite IETF |rfc=1930 |title =Guidelines for creation, selection, and registration of an Autonomous System (AS) |section=3 | first1 = John |last1=Hawkinson |first2=Tony |last2=Bates |date=March 1996 |publisher=[[Internet Engineering Task Force|IETF]]}}</ref> Each AS is assigned an '''autonomous system number''' ('''ASN'''), for use in [[Border Gateway Protocol]] (BGP) routing. Autonomous System Numbers are assigned to [[Regional_Internet_registry#Local_Internet_registry|Local Internet Registries]] (LIRs) and end-user organizations by their respective [[Regional Internet registry|Regional Internet Registries]] (RIRs), which in turn receive blocks of ASNs for reassignment from the [[Internet Assigned Numbers Authority]] (IANA). The IANA also maintains a registry of ASNs which are reserved for private use (and should therefore not be announced to the global Internet).

Originally, the definition required control by a single entity, typically an [[Internet service provider]] (ISP) or a very large organization with independent connections to multiple networks, that adhered to a single and clearly defined routing policy.<ref>{{cite IETF |rfc=1771 |title=A Border Gateway Protocol 4 (BGP-4) |first1=Yakov |last1=Rekhter |first2=Tony |last2=Li |date=March 1995 |publisher=[[Internet Engineering Task Force|IETF]]}} (obsoleted by {{IETF RFC|4271}})</ref> In March 1996, the newer definition came into use because multiple organizations can run [[Border Gateway Protocol|BGP]] using private AS numbers to an [[ISP]] that connects all those organizations to the Internet. Even though there may be multiple autonomous systems supported by the ISP, the Internet only sees the routing policy of the ISP. That ISP must have an officially registered ASN.

Until 2007, AS numbers were defined as 16-bit integers, which allowed for a maximum of 65,536 assignments. Since then,<ref name=rfc4893>{{cite IETF |rfc=4893 |title=BGP Support for Four-octet AS Number Space |first1 =Quaizar |last1=Vohra |first2=Enke |last2=Chen |date=May 2007 |publisher=[[Internet Engineering Task Force|IETF]]}} (obsoleted by {{IETF RFC|6793}})</ref> the [[Internet Assigned Numbers Authority|IANA]] has begun to also assign 32-bit AS numbers to [[regional Internet registry|regional Internet registries]] (RIRs). These numbers are written preferably as simple integers, in a notation referred to as "asplain",<ref name=rfc5396>{{cite IETF |rfc=5396 |title=Textual Representation of Autonomous System (AS) Numbers |first1=Geoff |last1=Huston |first2=George |last2=Michaelson |date=December 2008 |publisher=[[Internet Engineering Task Force|IETF]]}}</ref> ranging from 0 to 4,294,967,295 ([[hexadecimal]] 0xFFFF FFFF). Or, alternatively, in the form called "asdot+" which looks like ''x.y'', where ''x'' and ''y'' are 16-bit numbers. Numbers of the form ''0.y'' are exactly the old 16-bit AS numbers. The special 16-bit ASN 23456 ("AS_TRANS")<ref name=rfc6793>{{cite IETF|rfc=6793|title=BGP Support for Four-Octet Autonomous System (AS) Number Space|author1=Q. Vohra|author2=E. Chen|date=December 2012|publisher=[[IETF]]}}</ref> was assigned by IANA as a placeholder for 32-bit ASN values for the case when 32-bit-ASN capable routers ("new BGP speakers") send BGP messages to routers with older BGP software ("old BGP speakers") which do not understand the new 32-bit ASNs.<ref>{{cite web |url= http://icons.apnic.net/display/ASN/Using+AS+23456 |title= Using AS 23456: How BGP Uses Conversion or Truncation For Compatibility |date= 2008-07-21 |access-date=2018-12-31 |url-status =dead |archive-url = https://web.archive.org/web/20161029182359/http://icons.apnic.net/display/ASN/Using+AS+23456 |archive-date= 2016-10-29}}</ref>

The first and last ASNs of the original 16-bit integers (0 and 65,535) and the last ASN of the 32-bit numbers (4,294,967,295) are reserved<ref name=rfc6483>{{cite IETF|rfc=6483|title=Validation of Route Origination Using the Resource Certificate Public Key Infrastructure (PKI) and Route Origin Authorizations (ROAs)|author1=G. Huston|author2=G. Michaelson|date=February 2012|publisher=[[IETF]]}}</ref><ref name=rfc7300>{{cite IETF|rfc=7300|bcp=6|title=Reservation of Last Autonomous System (AS) Numbers|author1=J. Haas|author2=J. Mitchell|date=July 2014|issn=2070-1721|publisher=[[IETF]]}}</ref><ref name=rfc7607>{{cite IETF|rfc=7607|title=Codification of AS 0 Processing|author1=W. Kumari|author2=R. Bush|author3=H. Schiller|author4=K. Patel|date=August 2015|publisher=[[IETF]]}}</ref> and should not be used by operators; AS0 is used by all five RIRs to invalidate unallocated space.<ref>{{Cite web|title=IRR explorer|url=https://irrexplorer.dashcare.nl/asn/AS0|access-date=2021-10-24|website=irrexplorer.dashcare.nl}}</ref> ASNs 64,496 to 64,511 of the original 16-bit range and 65,536 to 65,551 of the 32-bit range are reserved for use in documentation.<ref name=rfc5398>{{cite IETF|rfc=5398|title=Autonomous System (AS) Number Reservation for Documentation Use|author=G. Huston|date=December 2008|publisher=Network Working Group}}</ref> ASNs 64,512 to 65,534 of the original 16-bit AS range, and 4,200,000,000 to 4,294,967,294 of the 32-bit range are reserved for Private Use.<ref name=rfc6996>{{cite IETF|rfc=6996|bcp=6|title=Autonomous System (AS) Reservation for Private Use|author=J. Mitchell|date=July 2013|issn=2070-1721|publisher=[[IETF]]}}</ref>

The number of unique autonomous networks in the routing system of the Internet exceeded 5,000 in 1999, 30,000 in late 2008, 35,000 in mid-2010, 42,000 in late 2012, 54,000 in mid-2016 and 60,000 in early 2018.<ref>{{cite web| url= https://www.cidr-report.org/as2.0/|title=CIDR report | last1 =Bates |first1=Tony |last2=Smith |first2= Philip| access-date= 2018-12-31 |last3 =Huston|first3=Geoff}}</ref>
By December 2020, the number of allocated ASNs exceeded 100,000. {{as of|2025|post=,}} there are roughly 120,000 allocated ASNs.<ref>{{cite web |url=https://www-public.imtbs-tsp.eu/~maigron/RIR_Stats/RIR_Delegations/World/ASN-ByNb.html |title= World - Autonomous System Number statistics - Sorted by number |work= Regional Internet Registries Statistics |access-date= 2018-12-31}}</ref>

==Assignment==
AS numbers are assigned in blocks by [[Internet Assigned Numbers Authority]] (IANA) to [[Regional Internet registry|regional Internet registries]] (RIRs). The appropriate RIR then assigns ASNs to entities within its designated area from the block assigned by IANA. Entities wishing to receive an ASN must complete the application process of their RIR, LIR or upstream service provider<ref>{{cite web |url=https://www.afrinic.net/library/198-how-to-setup-a-lir- |website=www.afrinic.net |access-date=23 March 2020 | title=How to setup a LIR}}</ref><ref>{{cite web |last1=Mar 2017 |first1=Publication date: 14 |title=Autonomous System (AS) Number Assignment Policies |url=https://www.ripe.net/publications/docs/ripe-679 |website=RIPE Network Coordination Centre |access-date=23 March 2020}}</ref> and be approved before being assigned an ASN. Current IANA ASN assignments to RIRs can be found on the IANA website.<ref name="iana">{{cite web |url= https://www.iana.org/assignments/as-numbers/as-numbers.xhtml |title= Autonomous System (AS) Numbers |website=IANA.org |date=2018-12-07 |access-date=2018-12-31}}</ref> RIRs, as part of [[Number Resource Organization|NRO]], can revoke AS numbers as part of their [[Internet governance]] abilities.<ref>{{Cite web|title=Revocation of Internet Resources allocation|url=https://www.lacnic.net/en/web/lacnic/revocacion-de-recursos|access-date=2021-02-17|website=www.lacnic.net|language=en}}</ref>

There are other sources for more specific data:<!-- this could look better -->

* [[APNIC]]: https://ftp.apnic.net/stats/apnic/
* [[RIPE NCC]]: https://ftp.ripe.net/ripe/stats/
* [[AFRINIC]]: https://ftp.afrinic.net/pub/stats/afrinic/
* [[ARIN]]: https://ftp.arin.net/pub/stats/arin/
* [[LACNIC]]: https://ftp.lacnic.net/pub/stats/lacnic/

==ASN table==
A complete table of available 16-bit and 32-bit ASN:<ref name="iana" />

{| class="wikitable"
|-
!Number
!Bits
!Description
!Reference
|-
|0
|16
|Reserved for [[Resource Public Key Infrastructure|RPKI]] unallocated space invalidation<ref>{{Cite web|date=2020-09-02|title=Policy prop-132 (AS0 for unallocated space) deployed in service|url=https://blog.apnic.net/2020/09/02/policy-prop-132-as0-for-unallocated-space-deployed-in-service/|access-date=2020-09-12|website=APNIC Blog|language=en-US}}</ref>
|{{IETF RFC|6483}}, {{IETF RFC|7607|link=no}}
|-
|1–23455
|16
|Public ASNs
|
|-
|23456
|16
|Reserved for AS Pool Transition
|{{IETF RFC|6793|link=no}}
|-
|23457–64495
|16
|Public ASNs
|
|-
|64496–64511
|16
|Reserved for use in documentation and sample code
|{{IETF RFC|5398|link=no}}
|-
|64512–65534
|16
|Reserved for private use
|{{IETF RFC|1930|link=no}}, {{IETF RFC|6996|link=no}}
|-
|65535
|16
|Reserved
|{{IETF RFC|7300|link=no}}
|-
|65536–65551
|32
|Reserved for use in documentation and sample code
|{{IETF RFC|5398|link=no}}, {{IETF RFC|6793|link=no}}
|-
|65552–131071
|32
|Reserved
|
|-
|131072–4199999999
|32
|Public 32-bit ASNs
|
|-
|4200000000–4294967294
|32
|Reserved for private use
|{{IETF RFC|6996|link=no}}
|-
|4294967295
|32
|Reserved
|{{IETF RFC|7300|link=no}}
|}

==Types==
Autonomous systems (AS) can be grouped into four categories, depending on their connectivity and operating policy.

# ''[[multihoming|multihomed]]'': An AS that maintains connections to more than one other AS. This allows the AS to remain connected to the Internet in the event of a complete failure of one of their connections. However, unlike a transit AS, this type of AS would not allow traffic from one AS to pass through on its way to another AS.
# ''[[Stub network|stub]]'': An AS that is connected to only one other AS. This may be an apparent waste of an AS number if the network's routing policy is the same as its upstream AS's. However, the stub AS may have [[peering]] with other autonomous systems that is not reflected in public [[Looking Glass server|route-view servers]]. Specific examples include private interconnections in the financial and transportation sectors.
# ''[[Internet transit|transit]]'': An AS that acts as a router between two ASes is called a transit. Since not all ASes are directly connected with every other AS, a transit AS carries data traffic between one AS to another AS to which it has links.<ref name="Paul 2016 BGP" />
# ''[[Internet Exchange Point]]'' (IX or IXP): A physical infrastructure through which ISPs or [[content delivery network]]s (CDNs) exchange Internet traffic between their networks (autonomous systems). These are often groups of local ISPs that band together to exchange data by splitting the costs of a local networking hub, avoiding the higher costs (and bandwidth charges) of a Transit AS. IXP ASNs are usually transparent. By having presence in an IXP, ASes shorten the transit path to other participating ASes, thereby reducing [[network latency]] and improving [[round-trip delay]].<ref name="Paul 2016 BGP">{{cite web|title= Understanding Autonomous Systems: Routing and Peering|url=https://people.cs.rutgers.edu/~pxk/352/notes/autonomous_systems.html|first=Paul|last= Krzyzanowski|access-date=29 March 2022|date=21 March 2016|publisher=[[Rutgers University]], Department of Computer Science|archive-url=https://web.archive.org/web/20220303153053/https://people.cs.rutgers.edu/~pxk/352/notes/autonomous_systems.html|archive-date=3 March 2022|url-status=live}}</ref><ref>{{cite web|url=https://www.cloudflare.com/en-gb/learning/cdn/glossary/internet-exchange-point-ixp/|title=Glossary: Internet exchange point (IXP)|access-date=29 March 2022|publisher=[[CloudFlare]]|url-status=live|archive-url=https://ghostarchive.org/archive/20220330010306/https://www.cloudflare.com/en-gb/web/20211103131937/https://www.cloudflare.com/en-gb/learning/cdn/glossary/internet-exchange-point-ixp/|archive-date=30 March 2022}}</ref>

== AS-SET ==
Autonomous systems can be included in one or more AS-SETs, for example AS-SET of RIPE NCC "AS-12655" has AS1, AS2 and AS3 as its members,<ref>{{Cite web|title=AS-12655 - bgp.he.net|url=https://bgp.he.net/irr/as-set/AS-12655|access-date=2021-10-24|website=bgp.he.net}}</ref> but AS1 is also included in other sets in ARIN (AS-INCAPSULA) and APNIC (AS-IMCL). Another AS-SET sources can be RADB, LEVEL3 ([[tier 1 network]] now called [[Lumen Technologies]]) and also ARIN has ARIN-NONAUTH source of AS-SETs.<ref>{{Cite web|title=IRR explorer|url=https://irrexplorer.nlnog.net/asn/AS1|access-date=2022-01-30|website=irrexplorer.nlnog.net}}</ref> AS-SETs are created by network operators in an [[Internet Routing Registry]] (IRR), like other route objects, and can be included in other AS-SETs and even form cycles.<ref>{{Cite web|title=IRR explorer|url=https://irrexplorer.nlnog.net/as-set/AS-ROSTELECOM|access-date=2022-01-30|website=irrexplorer.nlnog.net}}</ref><ref>{{Cite web|title=IDIDB - Объекты AS-SET|url=https://www.ididb.ru/asset/#AS-ROSTELECOM|access-date=2021-10-24|website=www.ididb.ru}}</ref>

AS-SET names usually start with "AS-", but can also have a hierarchical name. For example, the administrator of AS 64500 may create an AS-SET called "AS64500:AS-UPSTREAMS", to avoid conflict with other similarly named AS-SETs.<ref>{{cite web|url=https://apps.db.ripe.net/docs/04.RPSL-Object-Types/02-Descriptions-of-Primary-Objects.html#description-of-the-as-set-object|title=Descriptions of Primary Objects - RIPE Database Docs|access-date=2023-04-05}}</ref>

AS-SETs are often used to simplify management of published routing policies. A routing policy is published in the IRR using "import" and "export" (or the newer "mp-import" and "mp-export") attributes, which each contain the source or destination AS number and the AS number imported or exported. Instead of single AS numbers, AS-SETs can be referenced in these attributes, which simplifies management of complex routing policies.

==See also==

* [[Administrative distance]]
* [[INOC-DBA]] – a hotline communications system between the network operations centers of major Autonomous Systems
* [[Internet Routing Registry]]
* [[PeeringDB]] – a freely available web-based database of networks that are interested in peering
* [[Routing Assets Database]] (RADB)

==References==
{{Reflist}}

==External links==
* [https://stat.ripe.net/ RIPEstat] – Internet Measurements and Analysis
* [https://www.radb.net/ Merit] [[Routing Assets Database|RADb]]
* [https://bgp.he.net Hurricane Electric BGP Toolkit]
* [[PeeringDB]] https://www.peeringdb.com/
* [https://www.robtex.com/ Robtex: Various kinds of research of IP numbers, Domain names, ASN, etc]
* [http://www.netsniff-ng.org/ astraceroute, an AS traceroute utility (part of netsniff-ng)]
* [https://www.apnic.net/get-ip/faqs/asn/ ASN FAQ]
* [https://www.cidr-report.org/as2.0/ CIDR and ASN assignment report]
* [http://www.bgplookingglass.com/list-of-autonomous-system-numbers Partial List of Autonomous system numbers]
* [https://stat.lookinglass.org Lookin'STAT Graph: number of Autonomous systems online]

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[[Category:Internet architecture]]