View source for IPv6

{{short description|Version 6 of the Internet Protocol}}
{{Update|part=RFC 8200 and RFC 8201|date=July 2017}}
{{Use dmy dates|date=September 2020}}
{{Infobox networking protocol
| title        = Internet Protocol version 6
| logo         =
| logo alt     =
| image        = [[File:IPv6 header-en.svg|class=skin-invert-image|300px|alt=Diagram of an IPV6 header]]
| image alt    = Diagram of an IPv6 header
| caption      = IPv6 header
| is stack     = yes
| abbreviation = IPv6
| purpose      = [[Internetworking]] protocol
| developer    = [[Internet Engineering Task Force]]
| date         = {{Start date and age|df=yes|1995|12|}}<!--Fill in: Year (4 digits), month and day (2 digits)-->
| based on     = [[IPv4]]
| influenced   =
| osilayer     = [[Network layer]]
| ports        =
| rfcs         = {{IETF RFC|2460|8200|plainlink=yes}}
| hardware     =
}}
{{Internet protocol suite}}
{{Internet history timeline}}

'''Internet Protocol version 6''' ('''IPv6''') is the most recent version of the [[Internet Protocol]] (IP), the [[communication protocol|communications protocol]] that provides an identification and location system for computers on networks and routes traffic across the [[Internet]]. IPv6 was developed by the [[Internet Engineering Task Force]] (IETF) to deal with the long-anticipated problem of [[IPv4 address exhaustion]], and was intended to replace [[IPv4]].<ref name="ipv6nz">{{cite web|url=https://www.ipv6.org.nz/ipv6-faqs/|title=FAQs|publisher=New Zealand IPv6 Task Force|access-date=26 October 2015|archive-date=29 January 2019|archive-url=https://web.archive.org/web/20190129005124/http://www.ipv6.org.nz/ipv6-faqs/|url-status=dead}}</ref> In December 1998, IPv6 became a Draft Standard for the IETF,<ref name="rfc2460"/> which subsequently ratified it as an [[Internet Standard]] on 14 July 2017.<ref name="rfc8200"/><ref>{{Cite web |last=Siddiqui |first=Aftab |date=17 July 2017 |title=RFC 8200 – IPv6 Has Been Standardized |url=https://www.internetsociety.org/blog/2017/07/rfc-8200-ipv6-has-been-standardized/ |url-status=live |archive-url=https://web.archive.org/web/20231023162212/https://www.internetsociety.org/blog/2017/07/rfc-8200-ipv6-has-been-standardized/ |archive-date=23 October 2023 |access-date=25 February 2018 |publisher=[[Internet Society]] }}</ref>

Devices on the Internet are assigned a unique [[IP address]] for identification and location definition. With the rapid growth of the Internet after commercialization in the 1990s, it became evident that far more addresses would be needed to connect devices than the 4,294,967,296 (2<sup>32</sup>) IPv4 address space had available. By 1998, the IETF had formalized the successor protocol, IPv6 which uses 128-[[bit]] addresses, theoretically allowing 2<sup>128</sup>, or 340,282,366,920,938,463,463,374,607,431,768,211,456 total addresses. The actual number is slightly smaller, as multiple ranges are reserved for special usage or completely excluded from general use. The two protocols are not designed to be [[interoperable]], and thus direct communication between them is impossible, complicating the move to IPv6. However, several [[IPv6 transition mechanism|transition mechanisms]] have been devised to rectify this.

IPv6 provides other technical benefits in addition to a larger addressing space. In particular, it permits hierarchical address allocation methods that facilitate [[route aggregation]] across the Internet, and thus limit the expansion of [[routing table]]s. The use of multicast addressing is expanded and simplified, and provides additional optimization for the delivery of services. Device mobility, security, and configuration aspects have been considered in the design of the protocol.

IPv6 addresses are represented as eight groups of four [[hexadecimal]] digits each, separated by colons. The full representation may be shortened according to specific rules; for example, {{IPaddr|2001:0db8:0000:0000:0000:8a2e:0370:7334}} becomes {{IPaddr|2001:db8::8a2e:370:7334}}.

{{Toc level|3}}

==Main features==
[[File:IPv6 address terminology-en.svg|thumb|upright=1.2|Glossary of terms used for IPv6 addresses]]

IPv6 is an [[Internet Layer]] protocol for [[Packet switching|packet-switched]] [[internetworking]] and provides end-to-end [[datagram]] transmission across multiple IP networks, closely adhering to the design principles developed in the previous version of the protocol, [[IPv4|Internet Protocol Version 4]] (IPv4).

In addition to offering more addresses, IPv6 also implements features not present in IPv4. It simplifies aspects of address configuration, network renumbering, and router announcements when changing network connectivity providers. It simplifies packet processing in routers by placing the responsibility for packet fragmentation in the end points. The IPv6 [[subnetwork|subnet]] size is standardized by fixing the size of the host identifier portion of an address to 64 bits.

The addressing architecture of IPv6 allows three different types of transmission: [[unicast]], [[anycast]] and [[multicast]].{{Ref RFC|4291}}<ref name="Rosen kernel networking">{{Cite book|title=Linux Kernel Networking: Implementation and Theory|first=Rami|last=Rosen|publisher=Apress|date=2014|isbn=9781430261971|location=New York|oclc=869747983}}</ref>{{rp|210}} IPv6 does not implement [[Broadcasting (networking)|broadcast]], and therefore has no notion of a [[broadcast address]].

==Motivation and origin==
===IPv4 address exhaustion===
{{Main|IPv4 address exhaustion}}

[[File:IPv4 address structure and writing systems-en.svg|thumb|upright=1.2|Decomposition of the dot-decimal [[IPv4 address]] representation to its binary notation]]

[[IPv4|Internet Protocol Version 4]] (IPv4) was the first publicly used version of the [[Internet Protocol]]. IPv4 was developed as a research project by the [[DARPA|Defense Advanced Research Projects Agency]] (DARPA), a [[United States Department of Defense]] [[government agency|agency]], before becoming the foundation for the [[Internet]] and the [[World Wide Web]]. IPv4 includes an addressing system that uses numerical identifiers consisting of 32 bits. These addresses are typically displayed in [[dot-decimal notation]] as decimal values of four octets, each in the range 0 to 255, or 8 bits per number. Thus, IPv4 provides an addressing capability of 2<sup>32</sup> or approximately 4.3 billion addresses. Address exhaustion was not initially a concern in IPv4 as this version was originally presumed to be a test of DARPA's networking concepts.<ref>{{cite video|title=Google IPv6 Conference 2008: What will the IPv6 Internet look like?|url=https://www.youtube.com/watch?v=mZo69JQoLb8|archive-url=https://ghostarchive.org/varchive/youtube/20211211/mZo69JQoLb8|archive-date=2021-12-11|url-status=live|time=13:35}}{{cbignore}}</ref> During the first decade of operation of the Internet, it became apparent that methods had to be developed to conserve address space. In the early 1990s, even after the redesign of the addressing system using a [[Classless Inter-Domain Routing|classless network]] model, it became clear that this would not suffice to prevent [[IPv4 address exhaustion]], and that further changes to the Internet infrastructure were needed.<ref name=rfc1752>{{cite IETF|title=The Recommendation for the IP Next Generation Protocol|rfc=1752|first=S.|last=Bradner|first2=A.|last2=Mankin|date=January 1995|publisher=[[Internet Engineering Task Force|IETF]]}}</ref>

The last unassigned top-level address blocks of 16 million IPv4 addresses were allocated in February 2011 by the [[Internet Assigned Numbers Authority]] (IANA) to the five [[regional Internet registry|regional Internet registries]] (RIRs).<ref>{{Cite web |date=3 February 2011 |title=Free Pool of IPv4 Address Space Depleted |url=https://www.nro.net/ipv4-free-pool-depleted |url-status=live |archive-url=https://web.archive.org/web/20240118044214/https://www.nro.net/ipv4-free-pool-depleted |archive-date=18 January 2024 |access-date=19 January 2022 |website=NRO.net |publisher=The Number Resource Organization |location=[[Montevideo]] }}</ref> However, each RIR still has available address pools and is expected to continue with standard address allocation policies until one {{IPaddr|/8}} [[Classless Inter-Domain Routing]] (CIDR) block remains. After that, only blocks of 1,024 addresses (/22) will be provided from the RIRs to a [[local Internet registry]] (LIR). As of April 2025, all of [[Asia-Pacific Network Information Centre]] (APNIC), the [[RIPE NCC|Réseaux IP Européens Network Coordination Centre]] (RIPE NCC), [[Latin America and Caribbean Network Information Centre]] (LACNIC), [[AFRINIC|African Network Information Centre]] (AFRINIC), and [[American Registry for Internet Numbers]] (ARIN) have reached this stage.<ref>{{Cite web |last=Rashid |first=Fahmida |date=1 February 2011 |title=IPv4 Address Exhaustion Not Instant Cause for Concern with IPv6 in Wings |url=https://www.eweek.com/networking/ipv4-address-exhaustion-not-instant-cause-for-concern-with-ipv6-in-wings/ |url-status=live |archive-url=https://archive.today/20240120181901/https://www.eweek.com/networking/ipv4-address-exhaustion-not-instant-cause-for-concern-with-ipv6-in-wings/ |archive-date=20 January 2024 |access-date=23 June 2012 |publisher=eWeek }}</ref><ref>{{Cite news |last=Ward |first=Mark |date=14 September 2012 |title=Europe hits old internet address limits |url=https://www.bbc.co.uk/news/technology-19600718 |url-status=live |archive-url=https://web.archive.org/web/20231105171900/https://www.bbc.com/news/technology-19600718 |archive-date=5 November 2023 |access-date=15 September 2012 |work=[[BBC News]] }}</ref><ref>{{Cite web |last=Huston |first=Geoff |title=IPV4 Address Report |url=https://www.potaroo.net/tools/ipv4/ |url-status=live |archive-url=https://web.archive.org/web/20240110052921/https://www.potaroo.net/tools/ipv4/ |archive-date=10 January 2024 }}</ref><ref>{{Cite web |last=AFRINIC |date=2020-01-13 |title=AFRINIC enters IPv4 Exhaustion Phase 2 |url=https://afrinic.net/20200113-afrinic-enters-ipv4-exhaustion-phase-2 |access-date=2025-04-20 |website=afrinic.net |language=en-gb}}</ref>

RIPE NCC announced that it had fully run out of IPv4 addresses on 25 November 2019,<ref>{{Cite press release |date=25 November 2019 |title=The RIPE NCC has run out of IPv4 Addresses |url=https://www.ripe.net/publications/news/about-ripe-ncc-and-ripe/the-ripe-ncc-has-run-out-of-ipv4-addresses |url-status=live |archive-url=https://web.archive.org/web/20240119220002/https://www.ripe.net/publications/news/the-ripe-ncc-has-run-out-of-ipv4-addresses/ |archive-date=19 January 2024 |access-date=26 November 2019 |publisher=[[RIPE NCC]] }}</ref> and called for greater progress on the adoption of IPv6.

==Comparison with IPv4==
On the Internet, data is transmitted in the form of [[network packet]]s. IPv6 specifies a new [[IPv6 packet|packet format]], designed to minimize packet header processing by routers.{{Ref RFC|2460}}{{Ref RFC|1726}} Because the headers of IPv4 packets and IPv6 packets are significantly different, the two protocols are not interoperable. However, most transport and application-layer protocols need little or no change to operate over IPv6; exceptions are application protocols that embed Internet-layer addresses, such as [[File Transfer Protocol]] (FTP) and [[Network Time Protocol]] (NTP), where the new address format may cause conflicts with existing protocol syntax.

===Larger address space===
The main advantage of IPv6 over IPv4 is its larger address space. The size of an IPv6 address is 128 bits, compared to 32 bits in IPv4.<ref name=rfc2460/> The address space therefore has 2<sup>128</sup>=340,282,366,920,938,463,463,374,607,431,768,211,456 addresses (340 [[undecillion]], approximately {{val|3.4|e=38}}). Some blocks of this space and some specific addresses are [[reserved IP addresses|reserved for special uses]].

While this address space is very large, it was not the intent of the designers of IPv6 to assure geographical saturation with usable addresses. Rather, the longer addresses simplify allocation of addresses, enable efficient [[route aggregation]], and allow implementation of special addressing features. In IPv4, complex [[Classless Inter-Domain Routing]] (CIDR) methods were developed to make the best use of the small address space. The standard size of a subnet in IPv6 is 2<sup>64</sup> addresses, about four billion times the size of the entire IPv4 address space. Thus, actual address space utilization will be small in IPv6, but network management and routing efficiency are improved by the large subnet space and hierarchical route aggregation.

===Multicasting===
[[File:IPv6 multicast address stracture-en.svg|thumb|Multicast structure in IPv6]]
[[Multicast]]ing, the transmission of a packet to multiple destinations in a single send operation, is part of the base specification in IPv6. In IPv4 this is an optional (although commonly implemented) feature.{{Ref RFC|1112}} IPv6 multicast addressing has features and protocols in common with IPv4 multicast, but also provides changes and improvements by eliminating the need for certain protocols. IPv6 does not implement traditional [[Broadcast IP address|IP broadcast]], i.e. the transmission of a packet to all hosts on the attached link using a special ''broadcast address'', and therefore does not define broadcast addresses. In IPv6, the same result is achieved by sending a packet to the link-local ''all nodes'' multicast group at address {{IPaddr|ff02::1}}, which is analogous to IPv4 multicasting to address {{IPaddr|224.0.0.1}}. IPv6 also provides for new multicast implementations, including embedding rendezvous point addresses in an IPv6 multicast group address, which simplifies the deployment of inter-domain solutions.{{Ref RFC|3956}}

In IPv4 it is very difficult for an organization to get even one globally routable multicast group assignment, and the implementation of inter-domain solutions is arcane.{{Ref RFC|2908}} Unicast address assignments by a [[local Internet registry]] for IPv6 have at least a 64-bit routing prefix, yielding the smallest subnet size available in IPv6 (also 64 bits). With such an assignment it is possible to embed the unicast address prefix into the IPv6 multicast address format, while still providing a 32-bit block, the least significant bits of the address, or approximately 4.2 billion multicast group identifiers. Thus each user of an IPv6 subnet automatically has available a set of globally routable source-specific multicast groups for multicast applications.{{Ref RFC|3306}}

===Stateless address autoconfiguration (SLAAC)===
{{See also|IPv6 address#Stateless address autoconfiguration (SLAAC)|l1=IPv6 address § Stateless address autoconfiguration}}

IPv6 hosts configure themselves automatically. Every interface has a self-generated link-local address and, when connected to a network, conflict resolution is performed and routers provide network prefixes via router advertisements.{{Ref RFC|4862}} Stateless configuration of routers can be achieved with a special router renumbering protocol.{{Ref RFC|2894}} When necessary, hosts may configure additional stateful addresses via [[DHCPv6|Dynamic Host Configuration Protocol version 6]] (DHCPv6) or static addresses manually.

Like IPv4, IPv6 supports globally unique [[IP address]]es. The design of IPv6 intended to re-emphasize the end-to-end principle of network design that was originally conceived during the establishment of the early Internet by rendering [[network address translation]] obsolete. Therefore, every device on the network is globally addressable directly from any other device.

A stable, unique, globally addressable IP address would facilitate tracking a device across networks. Therefore, such addresses are a particular privacy concern for mobile devices, such as laptops and cell phones.<ref>{{cite web|url=https://www.internetsociety.org/resources/deploy360/2014/privacy-extensions-for-ipv6-slaac/|title=Privacy Extensions for Stateless Address Autoconfiguration in IPv6|author=T. Narten|author2=R. Draves|author3=S. Krishnan|date=September 2007|website=www.ietf.org|access-date=13 March 2017}}</ref> To address these privacy concerns, the SLAAC protocol includes what are typically called "privacy addresses" or, more correctly, "temporary addresses".{{Ref RFC|8981}} Temporary addresses are random and unstable. A typical consumer device generates a new temporary address daily and will ignore traffic addressed to an old address after one week. Temporary addresses are used by default by Windows since XP SP1,<ref>{{Cite web |title=Overview of the Advanced Networking Pack for Windows XP |url=http://support.microsoft.com/kb/817778 |url-status=dead |archive-url=https://web.archive.org/web/20170907013704/https://support.microsoft.com/en-us/help/817778/overview-of-the-advanced-networking-pack-for-windows-xp |archive-date=7 September 2017 |access-date=15 April 2019 |publisher=[[Microsoft]] }}</ref> macOS since (Mac&nbsp;OS&nbsp;X) 10.7, Android since 4.0, and iOS since version 4.3. Use of temporary addresses by Linux distributions varies.<ref>{{Cite web |date=8 August 2014 |title=Privacy Extensions for IPv6 SLAAC |url=https://www.internetsociety.org/resources/deploy360/2014/privacy-extensions-for-ipv6-slaac |url-status=live |archive-url=https://web.archive.org/web/20231023063407/https://www.internetsociety.org/resources/deploy360/2014/privacy-extensions-for-ipv6-slaac/ |archive-date=23 October 2023 |access-date=17 January 2020 |publisher=[[Internet Society]] }}</ref>

Renumbering an existing network for a new connectivity provider with different routing prefixes is a major effort with IPv4.<ref>{{Cite web |last1=Ferguson |first1=P. |last2=Berkowitz |first2=H. |date=January 1997 |title=Network Renumbering Overview: Why would I want it and what is it anyway? |url=https://datatracker.ietf.org/doc/html/rfc2071 |url-status=live |archive-url=https://web.archive.org/web/20240107145323/https://datatracker.ietf.org/doc/html/rfc2071 |archive-date=7 January 2024 |publisher=[[IETF]] |doi=10.17487/RFC2071 |rfc=2071 }}</ref><ref>{{Cite web |last=Berkowitz |first=H. |date=January 1997 |title=Router Renumbering Guide |url=https://datatracker.ietf.org/doc/html/rfc2072 |url-status=live |archive-url=https://web.archive.org/web/20230608094931/https://datatracker.ietf.org/doc/html/rfc2072 |archive-date=8 June 2023 |publisher=[[IETF]] |doi=10.17487/RFC2072 |rfc=2072 }}</ref> With IPv6, however, changing the prefix announced by a few routers can in principle renumber an entire network, since the host identifiers (the least-significant 64 bits of an address) can be independently self-configured by a host.{{Ref RFC|4862}}

The SLAAC address generation method is implementation-dependent. IETF recommends that addresses be deterministic but semantically opaque.<ref>{{Cite IETF|rfc=8064|title=Recommendation on Stable IPv6 Interface Identifiers|first1=Alissa|last1=Cooper|first2=Fernando|last2=Gont|first3=Dave|last3=Thaler}}</ref>

===IPsec===
[[Internet Protocol Security]] (IPsec) was originally developed for IPv6, but found widespread deployment first in IPv4, for which it was re-engineered. IPsec was a mandatory part of all IPv6 protocol implementations,<ref name=rfc2460/> and [[Internet Key Exchange]] (IKE) was recommended, but with RFC 6434 the inclusion of IPsec in IPv6 implementations was downgraded to a recommendation because it was considered impractical to require full IPsec implementation for all types of devices that may use IPv6.{{ref RFC|6434|quote=Previously, IPv6 mandated implementation of IPsec and recommended the key management approach of IKE.  This document updates that recommendation by making support of the IPsec Architecture RFC4301 a SHOULD for all IPv6 nodes. |p=17}} However, as of RFC 4301 IPv6 protocol implementations that do implement IPsec need to implement IKEv2 and need to support a minimum set of [[Cryptography|cryptographic algorithms]]. This requirement will help to make IPsec implementations more interoperable between devices from different vendors. The IPsec Authentication Header (AH) and the Encapsulating Security Payload header (ESP) are implemented as IPv6 extension headers.<ref>{{Cite book|title=IPv6 Essentials: Integrating IPv6 into Your IPv4 Network|author=Silvia Hagen|publisher=O'Reilly Media|year=2014|isbn=978-1-4493-3526-7|page=196|edition=3rd|location=Sebastopol, CA|oclc=881832733}}</ref>

===Simplified processing by routers===
The packet header in IPv6 is simpler than the IPv4 header. Many rarely used fields have been moved to optional header extensions. The IPv6 packet header has simplified the process of packet forwarding by [[Router (computing)|routers]]. Although IPv6 packet headers are at least twice the size of IPv4 packet headers, processing of packets that only contain the base IPv6 header by routers may, in some cases, be more efficient, because less processing is required in routers due to the headers being aligned to match common [[Word (computer architecture)|word sizes]].<ref name=rfc2460/><ref name=rfc1726/> However, many devices implement IPv6 support in software (as opposed to hardware), thus resulting in very bad packet processing performance.<ref>{{cite web|title=IPv6 Security Assessment and Benchmarking|first=E.|last=Zack|date=July 2013|url=http://www.ipv6hackers.org/meetings/ipv6-hackers-1}}</ref> Additionally, for many implementations, the use of Extension Headers causes packets to be processed by a router's CPU, leading to poor performance or even security issues.<ref name="draft-gont-v6ops-ipv6-ehs-packet-drops-03">{{Cite web |last=Gont |first=F. |date=March 2016 |title=Operational Implications of IPv6 Packets with Extension Headers |url=https://datatracker.ietf.org/doc/html/draft-gont-v6ops-ipv6-ehs-packet-drops-03 |url-status=live |archive-url=https://web.archive.org/web/20231027170015/https://datatracker.ietf.org/doc/html/draft-gont-v6ops-ipv6-ehs-packet-drops-03 |archive-date=27 October 2023 |publisher=[[IETF]] }}</ref>

Moreover, an IPv6 header does not include a checksum. The [[IPv4 header checksum]] is calculated for the IPv4 header, and has to be recalculated by routers every time the [[time to live]] (called [[hop limit]] in the IPv6 protocol) is reduced by one. The absence of a checksum in the IPv6 header furthers the [[end-to-end principle]] of Internet design, which envisioned that most processing in the network occurs in the leaf nodes. Integrity protection for the data that is encapsulated in the IPv6 packet is assumed to be assured by both the [[link layer]] or error detection in higher-layer protocols, namely the [[Transmission Control Protocol]] (TCP) and the [[User Datagram Protocol]] (UDP) on the [[transport layer]]. Thus, while IPv4 allowed UDP datagram headers to have no checksum (indicated by 0 in the header field), IPv6 requires a checksum in UDP headers.

IPv6 routers do not perform [[IP fragmentation]]. IPv6 hosts are required to do one of the following: perform [[Path MTU Discovery]], perform end-to-end fragmentation, or send packets no larger than the default [[maximum transmission unit]] (MTU), which is 1280 [[octet (computing)|octets]].

===Mobility===
Unlike mobile IPv4, [[mobile IPv6]] avoids [[triangular routing]] and is therefore as efficient as native IPv6. IPv6 routers may also allow entire subnets to move to a new router connection point without renumbering.{{Ref RFC|3963}}

===Extension headers===
[[File:IPv6 headers sequence-en.svg|thumb|Several examples of IPv6 extension headers]]

The IPv6 packet header has a minimum size of 40 octets (320 bits). Options are implemented as extensions. This provides the opportunity to extend the protocol in the future without affecting the core packet structure.<ref name="rfc2460"/> However, RFC 7872 notes that some network operators drop IPv6 packets with extension headers when they traverse transit [[Autonomous system (Internet)|autonomous systems]].

====Jumbograms====
IPv4 limits packets to 65,535 {{nowrap|(2<sup>16</sup> − 1)}} octets of payload. An IPv6 node can optionally handle packets over this limit, referred to as [[jumbogram]]s, which can be as large as 4,294,967,295 {{nowrap|(2<sup>32</sup> − 1)}} octets. The use of jumbograms may improve performance over high-[[Maximum transmission unit|MTU]] links. The use of jumbograms is indicated by the Jumbo Payload Option extension header.{{Ref RFC|2675}}

==IPv6 packets==
{{Main|IPv6 packet}}
[[File:Ipv6 header.svg|thumb|IPv6 packet header]]

An IPv6 packet has two parts: a [[Header (computing)|header]] and [[Payload (computing)|payload]].

The header consists of a fixed portion with minimal functionality required for all packets and may be followed by optional extensions to implement special features.

The fixed header occupies the first 40&nbsp;[[octet (computing)|octets]] (320 bits) of the IPv6 packet. It contains the source and destination addresses, traffic class, hop count, and the type of the optional extension or payload which follows the header. This ''Next Header'' field tells the receiver how to interpret the data which follows the header. If the packet contains options, this field contains the option type of the next option. The "Next Header" field of the last option points to the upper-layer protocol that is carried in the packet's [[payload (computing)|payload]].

The current use of the IPv6 Traffic Class field divides this between a 6 bit [[Differentiated Services Code Point]].{{Ref RFC|2474}} and a 2-bit [[Explicit Congestion Notification]] field.{{Ref RFC|3168}}

Extension headers carry options that are used for special treatment of a packet in the network, e.g., for routing, fragmentation, and for security using the [[IPsec]] framework.

Without special options, a payload must be less than {{gaps|64|kB}}. With a Jumbo Payload option (in a ''Hop-By-Hop Options'' extension header), the payload must be less than 4&nbsp;GB.

Unlike with IPv4, routers never fragment a packet. Hosts are expected to use [[Path MTU Discovery]] to make their packets small enough to reach the destination without needing to be fragmented. See [[IPv6 packet#Fragmentation|IPv6 packet fragmentation]].

==Addressing==
{{Main|IPv6 address}}

[[File:IPv6 address stracture-en.svg|thumb|A general structure for an IPv6 unicast address]]
[[IPv6 address]]es have 128 bits. The design of the IPv6 address space implements a different design philosophy than in IPv4, in which subnetting was used to improve the efficiency of utilization of the small address space. In IPv6, the address space is deemed large enough for the foreseeable future, and a local area subnet always uses 64 bits for the host portion of the address, designated as the interface identifier, while the most-significant 64 bits are used as the routing prefix.{{Ref RFC|4291|rp=9}} While the myth has existed regarding IPv6 subnets being impossible to scan, {{IETF RFC|7707}} notes that patterns resulting from some IPv6 address configuration techniques and algorithms allow address scanning in many real-world scenarios.

===Address representation===
The 128 bits of an IPv6 address are represented in 8 groups of 16 bits each. Each group is written as four hexadecimal digits (sometimes called ''[[hextet]]s''<ref name="Graziani2012">{{cite book|first=Rick|last=Graziani|title=IPv6 Fundamentals: A Straightforward Approach to Understanding IPv6|date=2012|publisher=[[Cisco Press]]|isbn=978-0-13-303347-2|page=55|url=https://books.google.com/books?id=FbYjJjZNA5gC&pg=PA55}}</ref><ref name="Coffeen2014">{{cite book|first=Tom|last=Coffeen|title=IPv6 Address Planning: Designing an Address Plan for the Future|date=2014|publisher=[[O'Reilly Media]]|isbn=978-1-4919-0326-1|page=170|url=https://books.google.com/books?id=dZU8BQAAQBAJ&pg=PT170}}</ref> or more formally ''[[hexadectet]]s''<ref name="Horley2013">{{cite book|first=Edward|last=Horley|title=Practical IPv6 for Windows Administrators|date=2013|publisher=[[Apress]]|isbn=978-1-4302-6371-5|page=17|url=https://books.google.com/books?id=u50QAwAAQBAJ&q=17&pg=PA17}}</ref> and informally a ''quibble'' or ''quad-nibble''<ref name="Horley2013"/>) and the groups are separated by colons (:). An example of this representation is {{IPaddr|2001:0db8:0000:0000:0000:ff00:0042:8329}}.

For convenience and clarity, the representation of an IPv6 address may be shortened with the following rules:
*One or more [[leading zero]]s from any group of hexadecimal digits are removed, which is usually done to all of the leading zeros. For example, the group {{IPaddr|0042}} is converted to {{IPaddr|42}}. The group {{IPaddr|0000}} is converted to {{IPaddr|0}}.
*Consecutive sections of zeros are replaced with two colons (::). This may only be used once in an address, as multiple use would render the address indeterminate. A double colon should not be used to denote an omitted single section of zeros.{{Ref RFC|5952|rsection=4.2.2}}

An example of application of these rules:
:Initial address: {{IPaddr|2001:0db8:0000:0000:0000:ff00:0042:8329}}.
:After removing all leading zeros in each group: {{IPaddr|2001:db8:0:0:0:ff00:42:8329}}.
:After omitting consecutive sections of zeros: {{IPaddr|2001:db8::ff00:42:8329}}.

The loopback address is defined as {{IPaddr|0000:0000:0000:0000:0000:0000:0000:0001}}{{Ref RFC|5156}} and is abbreviated to {{IPaddr|::1}} by using both rules.

As an IPv6 address may have more than one representation, the IETF has issued a [[IPv6 address#Representation|proposed standard for representing them in text]].{{Ref RFC|5952}}

Because IPv6 addresses contain colons, and URLs use colons to separate the host from the port number, an IPv6 address used as the host-part of a URL should be enclosed in square brackets,{{Ref RFC|3986}} e.g. <nowiki>http://[2001:db8:4006:812::200e]</nowiki> or <nowiki>http://[2001:db8:4006:812::200e]:8080/path/page.html</nowiki>.

===Link-local address===
[[File:IPv6 link local unicast address structure-en.svg|thumb|The Link-Local Unicast Address structure in IPv6]]

All interfaces of IPv6 hosts require a [[link-local address]], which have the prefix {{IPaddr|fe80::|10}}. This prefix is followed by 54 bits that can be used for subnetting, although they are typically set to zeros, and a 64-bit interface identifier. The host can compute and assign the Interface identifier by itself without the presence or cooperation of an external network component like a DHCP server, in a process called ''link-local address autoconfiguration''.{{Citation needed|date=January 2022}}

The lower 64 bits of the link-local address (the suffix) were originally derived from the MAC address of the underlying network interface card. As this method of assigning addresses would cause undesirable address changes when faulty network cards were replaced, and as it also suffered from a number of security and privacy issues, {{IETF RFC|8064}} has replaced the original MAC-based method with the hash-based method specified in {{IETF RFC|7217}}.{{Citation needed|date=January 2022}}

===Address uniqueness and router solicitation===
IPv6 uses a new mechanism for mapping IP addresses to link-layer addresses (e.g. [[MAC address]]es), because it does not support the [[Broadcasting (networking)|broadcast]] addressing method, on which the functionality of the [[Address Resolution Protocol]] (ARP) in IPv4 is based. IPv6 implements the [[Neighbor Discovery Protocol]] (NDP, ND) in the [[link layer]], which relies on [[ICMPv6]] and [[multicast]] transmission.<ref name="Rosen kernel networking"/>{{rp|210}} IPv6 hosts verify the uniqueness of their IPv6 addresses in a [[local area network]] (LAN) by sending a neighbor solicitation message asking for the link-layer address of the IP address. If any other host in the LAN is using that address, it responds.<ref name="T. Narten pp. 54">{{cite journal|first=T.|last=Narten|title=Neighbor discovery and stateless autoconfiguration in IPv6|journal=IEEE Internet Computing|volume=3|issue=4|pages=54–62|date=August 1999|doi=10.1109/4236.780961}}</ref>

A host bringing up a new IPv6 interface first generates a unique link-local address using one of several mechanisms designed to generate a unique address. Should a non-unique address be detected, the host can try again with a newly generated address. Once a unique link-local address is established, the IPv6 host determines whether the LAN is connected on this link to any [[Router (computing)|router]] interface that supports IPv6. It does so by sending out an ICMPv6 router solicitation message to the all-routers<ref name="rfc4861sec637">{{Cite web |last=Narten |first=T. |date=September 2007 |title=Neighbor Discovery for IP version 6 (IPv6) |url=https://datatracker.ietf.org/doc/html/rfc4861#section-6.3.7 |url-status=live |archive-url=https://web.archive.org/web/20240117035643/https://datatracker.ietf.org/doc/html/rfc4861#section-6.3.7 |archive-date=17 January 2024 |publisher=[[IETF]] |at=section 6.3.7 |doi=10.17487/RFC4861 |rfc=4861 |doi-access=free }}</ref> multicast group with its link-local address as source. If there is no answer after a predetermined number of attempts, the host concludes that no routers are connected. If it does get a response, known as a router advertisement, from a router, the response includes the network configuration information to allow establishment of a globally unique address with an appropriate unicast network prefix.<ref name="rfc4862sec551">{{Cite web |last=Thomson |first=S. |date=September 2007 |title=IPv6 Stateless Address Autoconfiguration - Section 5.5.1 |url=https://datatracker.ietf.org/doc/html/rfc4862#section-5.5.1 |url-status=live |archive-url=https://web.archive.org/web/20240111084216/https://datatracker.ietf.org/doc/html/rfc4862#section-5.5.1 |archive-date=11 January 2024 |publisher=[[IETF]] |doi=10.17487/RFC4862 |rfc=4862 }}</ref> There are also two flag bits that tell the host whether it should use DHCP to get further information and addresses:
*The Manage bit, which indicates whether or not the host should use DHCP to obtain additional addresses rather than rely on an auto-configured address from the router advertisement.
*The Other bit, which indicates whether or not the host should obtain other information through DHCP. The other information consists of one or more prefix information options for the subnets that the host is attached to, a lifetime for the prefix, and two flags:<ref name="T. Narten pp. 54"/>
**On-link: If this flag is set, the host will treat all addresses on the specific subnet as being on-link and send packets directly to them instead of sending them to a router for the duration of the given lifetime.
**Address: This flag tells the host to actually create a global address.

===Global addressing===
[[File:IPv6 global unicast address stracture-en.svg|thumb|The global unicast address structure in IPv6]]
The assignment procedure for global addresses is similar to local-address construction. The prefix is supplied from router advertisements on the network. Multiple prefix announcements cause multiple addresses to be configured.<ref name="T. Narten pp. 54"/>

Stateless address autoconfiguration (SLAAC) requires a {{IPaddr||64}} address block.{{Ref RFC|4291}} [[Local Internet registry|Local Internet registries]] are assigned at least {{IPaddr||32}} blocks, which they divide among subordinate networks.<ref>{{Cite web |date=8 February 2011 |title=IPv6 Address Allocation and Assignment Policy |url=https://www.ripe.net/publications/docs/ripe-512/ |url-status=live |archive-url=https://web.archive.org/web/20230603052402/https://www.ripe.net/publications/docs/ripe-512 |archive-date=3 June 2023 |access-date=27 March 2011 |publisher=[[RIPE NCC]] }}</ref> The initial recommendation of {{date|September 2001}} stated assignment of a {{IPaddr||48}} subnet to end-consumer sites.{{Ref RFC|3177}} In {{Date|March 2011}} this recommendation was refined:{{Ref RFC|6177}} The [[IETF]] "recommends giving home sites significantly more than a single {{IPaddr||64}}, but does not recommend that every home site be given a {{IPaddr||48}} either". Blocks of {{IPaddr||56}}s are specifically considered. It remains to be seen whether ISPs will honor this recommendation. For example, during initial trials, [[Comcast]] customers were given a single {{IPaddr||64}} network.<ref>{{Cite press release |last=Brzozowski |first=John |date=31 January 2011 |title=Comcast Activates First Users With IPv6 Native Dual Stack Over DOCSIS |url=https://corporate.comcast.com/comcast-voices/comcast-activates-first-users-with-ipv6-native-dual-stack-over-docsis |url-status=live |archive-url=https://web.archive.org/web/20231023064638/https://corporate.comcast.com/comcast-voices/comcast-activates-first-users-with-ipv6-native-dual-stack-over-docsis |archive-date=23 October 2023 |access-date=15 April 2019 |publisher=[[Comcast]] }}</ref>

==IPv6 in the Domain Name System==
In the [[Domain Name System]] (DNS), [[hostname]]s are mapped to IPv6 addresses by [[AAAA record|AAAA]] ("quad-A") resource records. For [[Reverse DNS lookup|reverse resolution]], the IETF reserved the domain [[.arpa|ip6.arpa]], where the name space is hierarchically divided by the 1-digit [[hexadecimal]] representation of [[nibble]] units (4 bits) of the IPv6 address.{{Ref RFC|3596}}

When a dual-stack host queries a DNS server to resolve a [[fully qualified domain name]] (FQDN), the DNS client of the host sends two DNS requests, one querying AAAA records and the other querying A records, in that order, by default. If both types of addresses are returned by the DNS, and there is a route for it, the IPv6 address is preferred over the IPv4 address. However, the host operating system may be configured with an alternate preference for address selection.{{Ref RFC|6724}}<ref>{{Cite book|title=IPv6 Essentials: Integrating IPv6 into Your IPv4 Network|author=Silvia Hagen|publisher=O'Reilly Media, Inc.|year=2014|isbn=9781449335267|pages=176}}</ref>

<!--[[A6 record]] redirects to this section.-->
An alternative record type was used in early DNS implementations for IPv6, designed to facilitate network renumbering. The ''A6'' resource record was used for the forward lookup, completed with a number of other innovations such as ''bit-string labels'' and ''[[CNAME record#DNAME record|DNAME]]'' records.{{Ref RFC|2874}} After a discussion of the pros and cons of both schemes,{{Ref RFC|3364}}) the use of A6 resource records has been deprecated to experimental status.{{Ref RFC|3363}}

{{See also|Happy eyeballs}}

==Transition mechanisms==
{{Main|IPv6 transition mechanism}}

IPv6 is not foreseen to supplant IPv4 instantaneously. Both protocols will continue to operate simultaneously for some time. Therefore, [[IPv6 transition mechanism]]s are needed to enable IPv6 hosts to reach IPv4 services and to allow isolated IPv6 hosts and networks to reach each other over IPv4 infrastructure.<ref name="sixxs">{{Cite web |title=IPv6 Transition Mechanism/Tunneling Comparison |url=https://www.sixxs.net/faq/connectivity/?faq=comparison |url-status=live |archive-url=https://web.archive.org/web/20231023064851/https://www.sixxs.net/faq/connectivity/?faq=comparison |archive-date=23 October 2023 |access-date=20 January 2012 |publisher=Sixxs.net }}</ref>

According to [[Silvia Hagen]], a dual-stack implementation of the IPv4 and IPv6 on devices is the easiest way to migrate to IPv6.<ref>{{Cite book|title=IPv6 Essentials: Integrating IPv6 into Your IPv4 Network|author=Silvia Hagen|publisher=O'Reilly Media, Inc.|year=2014|isbn=9781449335267|pages=222–223}}</ref> Many other transition mechanisms use tunneling to encapsulate IPv6 traffic within IPv4 networks and vice versa. This is an imperfect solution, which reduces the [[maximum transmission unit]] (MTU) of a link and therefore complicates [[Path MTU Discovery]], and may increase [[Network latency|latency]].<ref>{{Cite web |last1=Carpenter |first1=B. |date=August 2011 |title=Advisory Guidelines for 6to4 Deployment |url=https://datatracker.ietf.org/doc/html/rfc6343 |url-status=live |archive-url=https://web.archive.org/web/20230128112750/https://datatracker.ietf.org/doc/html/rfc6343 |archive-date=28 January 2023 |access-date=20 August 2012 |publisher=[[IETF]] |doi=10.17487/RFC6343 |rfc=6343 |doi-access=free }}</ref><ref>{{Cite web |date=5 September 2007 |title=IPv6: Dual stack where you can; tunnel where you must |url=https://www.networkworld.com/article/813230/ipv6-dual-stack-where-you-can-tunnel-where-you-must.html |url-status=live |archive-url=https://web.archive.org/web/20240120184843/https://www.networkworld.com/article/813230/ipv6-dual-stack-where-you-can-tunnel-where-you-must.html |archive-date=20 January 2024 |access-date=27 November 2012 |publisher=networkworld.com }}</ref>

===Dual-stack IP implementation===
Dual-stack IP implementations provide complete IPv4 and IPv6 protocol stacks in the operating system of a [[computer]] or [[network device]] on top of the common [[physical layer]] implementation, such as [[Ethernet]]. This permits dual-stack hosts to participate in IPv6 and IPv4 networks simultaneously.{{Ref RFC|4213}}

A device with dual-stack implementation in the operating system has an IPv4 and IPv6 address, and can communicate with other nodes in the LAN or the Internet using either IPv4 or IPv6. The DNS protocol is used by both IP protocols to resolve fully qualified domain names and IP addresses, but dual stack requires that the resolving DNS server can resolve both types of addresses. Such a dual-stack DNS server holds IPv4 addresses in the A records and IPv6 addresses in the AAAA records. Depending on the destination that is to be resolved, a DNS name server may return an IPv4 or IPv6 IP address, or both. A default address selection mechanism, or preferred protocol, needs to be configured either on hosts or the DNS server. The [[IETF]] has published [[Happy Eyeballs]] to assist dual-stack applications, so that they can connect using both IPv4 and IPv6, but prefer an IPv6 connection if it is available. However, dual-stack also needs to be implemented on all routers between the host and the service for which the DNS server has returned an IPv6 address. Dual-stack clients should be configured to prefer IPv6 only if the network is able to forward IPv6 packets using the IPv6 versions of [[routing protocols]]. When dual-stack network protocols are in place the [[application layer]] can be migrated to IPv6.<ref>{{Cite book|title=IPv6 Essentials: Integrating IPv6 into Your IPv4 Network|author=Silvia Hagen|publisher=O'Reilly Media, Inc.|year=2014|isbn=9781449335267|pages=222}}</ref>

While dual-stack is supported by major [[operating system]] and network device vendors, legacy networking hardware and servers do not support IPv6.

===ISP customers with public-facing IPv6===
[[File:IPv6 Prefix Assignment Example-en.svg|thumb|upright=1.2|IPv6 Prefix Assignment mechanism with IANA, RIRs, and ISPs]]

[[Internet service providers]] (ISPs) are increasingly providing their business and private customers with public-facing IPv6 global unicast addresses. If IPv4 is still used in the local area network (LAN), however, and the ISP can only provide one public-facing IPv6 address, the IPv4 LAN addresses are translated into the public facing IPv6 address using [[NAT64]], a [[network address translation]] (NAT) mechanism. Some ISPs cannot provide their customers with public-facing IPv4 and IPv6 addresses, thus supporting dual-stack networking, because some ISPs have exhausted their globally routable IPv4 address pool. Meanwhile, ISP customers are still trying to reach IPv4 [[web servers]] and other destinations.<ref>{{cite web|url=https://www.juniper.net/documentation/en_US/junos/topics/concept/ipv6-dual-stack-understanding.html|title=Understanding Dual Stacking of IPv4 and IPv6 Unicast Addresses|website=Juniper.net|publisher=Juniper Networks|date=31 August 2017|access-date=19 January 2022}}</ref>

A significant percentage of ISPs in all [[regional Internet registry]] (RIR) zones have obtained IPv6 address space. This includes many of the world's major ISPs and [[mobile network]] operators, such as [[Verizon Wireless]], [[StarHub|StarHub Cable]], [[Chubu Electric Power|Chubu Telecommunications]], [[Kabel Deutschland]], [[Swisscom]], [[T-Mobile International AG|T-Mobile]], [[Internode (ISP)|Internode]] and [[Telefónica]].<ref>{{cite web|url=https://www.nro.net/ipv6/|title=IPv6|website=NRO.net|access-date=13 March 2017|archive-date=12 January 2017|archive-url=https://web.archive.org/web/20170112052541/https://www.nro.net/ipv6|url-status=dead}}</ref>

While some ISPs still allocate customers only IPv4 addresses, many ISPs allocate their customers only an IPv6 or dual-stack IPv4 and IPv6. ISPs report the share of IPv6 traffic from customers over their network to be anything between 20% and 40%, but by mid-2017 IPv6 traffic still only accounted for a fraction of total traffic at several large [[Internet exchange point]]s (IXPs). [[AMS-IX]] reported it to be 2% and [[SeattleIX]] reported 7%. A 2017 survey found that many DSL customers that were served by a dual stack ISP did not request DNS servers to resolve fully qualified domain names into IPv6 addresses. The survey also found that the majority of traffic from IPv6-ready web-server resources were still requested and served over IPv4, mostly due to ISP customers that did not use the dual stack facility provided by their ISP and to a lesser extent due to customers of IPv4-only ISPs.<ref>{{Cite web |last=Pujol |first=Enric |date=12 June 2017 |title=What Stops IPv6 Traffic in a Dual-Stack ISP? |url=https://blog.apnic.net/2017/06/13/stops-ipv6-traffic-dual-stack-isp/ |url-status=live |archive-url=https://web.archive.org/web/20230327133355/https://blog.apnic.net/2017/06/13/stops-ipv6-traffic-dual-stack-isp/ |archive-date=27 March 2023 |access-date=13 June 2017 |website=APNIC.net |publisher=[[APNIC]] }}</ref>

===Tunneling===
The technical basis for tunneling, or encapsulating IPv6 packets in IPv4 packets, is outlined in RFC 4213. When the Internet backbone was IPv4-only, one of the frequently used tunneling protocols was [[6to4]].<ref name="Steven J. Vaughan-Nichols">{{Cite news |last=Vaughan-Nichols |first=Steven J. |date=14 October 2010 |title=Five ways for IPv6 and IPv4 to peacefully co-exist |url=https://www.zdnet.com/home-and-office/networking/five-ways-for-ipv6-and-ipv4-to-peacefully-co-exist/ |url-status=live |archive-url=https://web.archive.org/web/20231205094000/https://www.zdnet.com/home-and-office/networking/five-ways-for-ipv6-and-ipv4-to-peacefully-co-exist/ |archive-date=5 December 2023 |access-date=13 March 2017 |work=[[ZDNET]] }}</ref> [[Teredo tunneling]] was also frequently used for integrating IPv6 LANs with the IPv4 Internet backbone. Teredo is outlined in RFC 4380 and allows IPv6 [[local area networks]] to tunnel over IPv4 networks, by encapsulating IPv6 packets within UDP. The Teredo relay is an IPv6 router that mediates between a Teredo server and the native IPv6 network. It was expected that 6to4 and Teredo would be widely deployed until ISP networks would switch to native IPv6, but by 2014 Google Statistics showed that the use of both mechanisms had dropped to almost 0.<ref>{{Cite book|title=IPv6 Essentials: Integrating IPv6 into Your IPv4 Network|author=Silvia Hagen|publisher=O'Reilly Media, Inc.|year=2014|isbn=9781449335267|pages=33}}</ref>

===IPv4-mapped IPv6 addresses===
[[File:IPv6 IPv4-Compatible address structure-en.svg|thumb|IPv4-compatible IPv6 unicast address]]
[[File:IPv6 IPv4-Mapped address structure-en.svg|thumb|IPv4-mapped IPv6 unicast address]]

Hybrid dual-stack IPv6/IPv4 implementations recognize a special class of addresses, the IPv4-mapped IPv6 addresses.{{Ref RFC|6890|rsection=2.2.3}}{{Ref RFC|4291}} These addresses are typically written with a 96-bit prefix in the standard IPv6 format, and the remaining 32 bits are written in the customary [[dot-decimal notation]] of IPv4.

Addresses in this group consist of an 80-bit prefix of zeros, the next 16 bits are ones, and the remaining, least-significant 32 bits contain the IPv4 address. For example, {{IPaddr|::ffff:192.0.2.128}} represents the IPv4 address {{IPaddr|192.0.2.128}}. A previous format, called "IPv4-compatible IPv6 address", was {{IPaddr|::192.0.2.128}}; however, this method is deprecated.<ref name="rfc4291"/>

Because of the significant internal differences between IPv4 and IPv6 protocol stacks, some of the lower-level functionality available to programmers in the IPv6 stack does not work the same when used with IPv4-mapped addresses. Some common IPv6 stacks do not implement the IPv4-mapped address feature, either because the IPv6 and IPv4 stacks are separate implementations (e.g., [[Microsoft Windows]] 2000, XP, and Server 2003), or because of security concerns ([[OpenBSD]]).<ref name="openbsd-mapped-addr">{{man|4|inet6|OpenBSD}}</ref> On these operating systems, a program must open a separate socket for each IP protocol it uses. On some systems, e.g., the [[Linux kernel]], [[NetBSD]], and [[FreeBSD]], this feature is controlled by the socket option IPV6_V6ONLY.<ref name="rfc3493">{{cite IETF|rfc=3493|title=Basic Socket Interface Extensions for IPv6|author1=R. Gilligan|author2=S. Thomson|author3=J. Bound|author4=J. McCann|author5=W. Stevens|publisher=Network Working Group|date=February 2003}}</ref>{{rp|page=22}}

The address prefix {{IPaddr|64:ff9b::/96}} is a class of IPv4-embedded IPv6 addresses for use in [[NAT64]] transition methods.{{Ref RFC|6052}} For example, {{IPaddr|64:ff9b::192.0.2.128}} represents the IPv4 address {{IPaddr|192.0.2.128}}.<!--This needs a lot better explanation-->

==Security==
A number of security implications may arise from the use of IPv6. Some of them may be related with the IPv6 protocols themselves, while others may be related with implementation flaws.<ref>{{citation|title=IPv6 Security for IPv4 Engineers|url=https://www.internetsociety.org/wp-content/uploads/2019/03/deploy360-ipv6-security-v1.0.pdf|last=Gont|first=Fernando|date=10 March 2019|access-date=30 August 2019}}</ref><ref>{{citation|title=IPv6 Security Frequently Asked Questions (FAQ)|url=https://www.internetsociety.org/wp-content/uploads/2019/02/Deploy360-IPv6-Security-FAQ.pdf|last=Gont|first=Fernando|date=10 January 2019|access-date=30 August 2019}}</ref>

===Shadow networks===
The addition of nodes having IPv6 enabled by default by the software manufacturer may result in the inadvertent creation of ''shadow networks'', causing IPv6 traffic flowing into networks having only IPv4 security management in place. This may also occur with operating system upgrades, when the newer operating system enables IPv6 by default, while the older one did not. Failing to update the security infrastructure to accommodate IPv6 can lead to IPv6 traffic bypassing it.<ref>{{citation|title=Shadow Networks: an Unintended IPv6 Side Effect|archive-url=https://web.archive.org/web/20130411113334/http://www.networkcomputing.com/ipv6-tech-center/shadow-networks-an-unintended-ipv6-side/232800326|archive-date=11 April 2013|url=http://www.networkcomputing.com/ipv6-tech-center/shadow-networks-an-unintended-ipv6-side/232800326|last=Mullins|first=Robert|date=5 April 2012|access-date=2 March 2013}}</ref> Shadow networks have occurred on business networks in which enterprises are replacing [[Windows XP]] systems that do not have an IPv6 stack enabled by default, with [[Windows 7]] systems, that do.<ref>{{cite book|title=IPv6 For All: A Guide for IPv6 Usage and Application in Different Environments|url=https://www.ipv6forum.com/dl/books/ipv6forall.pdf|first1=Guillermo|last1=Cicileo|first2=Roque|last2=Gagliano|first3=Christian|last3=O’Flaherty|first4=Mariela|last4=Rocha|first5=César Olvera|last5=Morales|first6=Jordi Palet|last6=Martínez|first7=Álvaro Vives|last7=Martínez|display-authors=3|page=5|date=October 2009|access-date=2 March 2013}}</ref> Some IPv6 stack implementors have therefore recommended disabling IPv4 mapped addresses and instead using a dual-stack network where supporting both IPv4 and IPv6 is necessary.<ref>{{cite web|url=https://tools.ietf.org/html/draft-itojun-v6ops-v4mapped-harmful-02|title=IPv4-Mapped Addresses on the Wire Considered Harmful|author=Jun-ichiro itojun Hagino|date=October 2003}}</ref>

===IPv6 packet fragmentation===
Research has shown that the use of fragmentation could be leveraged to evade network security controls, similar to IPv4. As a result, it is now required that the first fragment of an IPv6 packet contains the entire IPv6 header chain,{{Ref RFC|7112}} such that some very pathological fragmentation cases are forbidden. Additionally, as a result of research on the evasion of RA-Guard, the use of fragmentation is deprecated with [[Neighbor Discovery]],{{Ref RFC|7113}} and discouraged with [[Secure Neighbor Discovery]] (SEND).{{Ref RFC|6980}}

==Standardization through RFCs==
===Working-group proposals===
[[file:IPv6 timeline-en.svg|thumb|A timeline for the standards governing IPv6]]
Due to the anticipated global growth of the [[Internet]], the [[Internet Engineering Task Force]] (IETF) in the early 1990s started an effort to develop a next generation IP protocol.<ref name="Rosen kernel networking"/>{{rp|209}} By the beginning of 1992, several proposals appeared for an expanded Internet addressing system and by the end of 1992 the IETF announced a call for white papers.<ref>{{cite web|rfc=1550|title=IP: Next Generation (IPng) White Paper Solicitation|first1=S.|last1=Bradner|first2=A.|last2=Mankin|date=December 1993|url=https://tools.ietf.org/html/rfc1550}}</ref> In September 1993, the IETF created a temporary, ad hoc ''IP Next Generation'' (IPng) area to deal specifically with such issues. The new area was led by [[Allison Mankin]] and [[Scott Bradner]], and had a directorate with 15 engineers from diverse backgrounds for direction-setting and preliminary document review:<ref name=rfc1752/><ref>{{cite web|url=http://grnlight.net/index.php/programming-articles/103-history-of-the-ipng-effort|archive-url=https://web.archive.org/web/20140523072903/http://grnlight.net/index.php/programming-articles/103-history-of-the-ipng-effort|archive-date=23 May 2014|work=The Sun|url-status=usurped|title=History of the IPng Effort}}</ref> The working-group members were [[J. Allard]] (Microsoft), [[Steven M. Bellovin|Steve Bellovin]] (AT&T), Jim Bound (Digital Equipment Corporation), Ross Callon (Wellfleet), [[Brian Carpenter (Internet engineer)|Brian Carpenter]] (CERN), [[David D. Clark|Dave Clark]] (MIT), [[John Curran (businessman)|John Curran]] (NEARNET), [[Steve Deering]] (Xerox), Dino Farinacci (Cisco), Paul Francis (NTT), Eric Fleischmann (Boeing), Mark Knopper (Ameritech), Greg Minshall (Novell), Rob Ullmann (Lotus), and [[Lixia Zhang]] (Xerox).<ref>{{cite web|rfc=1752|title=The Recommendation for the IP Next Generation Protocol – Appendix B|date=January 1995 |url=https://tools.ietf.org/html/rfc1752#appendix-B |last1=Bradner |first1=Scott O. |last2=Mankin |first2=Allison J. }}</ref>

The Internet Engineering Task Force adopted the IPng model on 25 July 1994, with the formation of several IPng working groups.<ref name=rfc1752/> By 1996, a series of [[Request for Comments|RFCs]] was released defining Internet Protocol version 6 (IPv6), starting with {{IETF RFC|1883}}. (Version 5 was used by the experimental [[Internet Stream Protocol]].)

===RFC standardization===
The first RFC to standardize IPv6 was the {{IETF RFC|1883}} in 1995.<ref>{{Cite journal |last1=Wang |first1=Tao |last2=Gao |first2=Jiaqiong |date=2019-01-01 |title=The Shortcomings of Ipv6 and Upgrade of Ipv4 |journal=International Journal of Advanced Network, Monitoring and Controls |language=en |volume=4 |issue=1 |pages=1–9 |doi=10.21307/ijanmc-2019-029|doi-access=free }}</ref> In 1998 {{IETF RFC|2460}} became the RFC for IPv6.<ref name="Rosen kernel networking"/>{{Rp|209}} In July 2017  [[rfc:2460|RFC 2460]] was superseded by {{IETF RFC|8200}}, which elevated IPv6 to "Internet Standard" (the highest maturity level for IETF protocols).{{Ref RFC|8200}}

==Deployment==
{{Main|IPv6 deployment}}
[[File:Rir-ipv6-allocation-rate.svg|thumb|Monthly IPv6 allocations per [[regional Internet registry]] (RIR)]]

The 1993 introduction of [[Classless Inter-Domain Routing]] (CIDR) in the routing and IP address allocation for the Internet, and the extensive use of [[network address translation]] (NAT), delayed [[IPv4 address exhaustion]] to allow for IPv6 deployment, which began in the mid-2000s.

Universities were among the early adopters of IPv6. [[Virginia Tech]] deployed IPv6 at a trial location in 2004 and later expanded IPv6 deployment across the [[campus network]]. By 2016, 82% of the traffic on their network used IPv6. [[Imperial College London]] began experimental IPv6 deployment in 2003 and by 2016 the IPv6 traffic on their networks averaged between 20% and 40%. A significant portion of this IPv6 traffic was generated through their [[high energy physics]] collaboration with [[CERN]], which relies entirely on IPv6.<ref>{{Citation|title=State of IPv6 Deployment 2018|url=https://www.internetsociety.org/resources/2018/state-of-ipv6-deployment-2018/|page=3|year=2018|publisher=[[Internet Society]]}}</ref>

The [[Domain Name System]] (DNS) has supported IPv6 since 2008. In the same year, IPv6 was first used in a major world event during the Beijing [[2008 Summer Olympics]].<ref name="beijing2008-pressrelease">{{cite press release|title=Beijing2008.cn leaps to next-generation Net|publisher=The Beijing Organizing Committee for the Games of the XXIX Olympiad|date=30 May 2008|url=http://en.beijing2008.cn/news/official/preparation/n214384681.shtml|url-status=dead|archive-url=https://web.archive.org/web/20090204051327/http://en.beijing2008.cn/news/official/preparation/n214384681.shtml|archive-date=4 February 2009}}</ref><ref>{{cite web|url=http://ipv6.com/articles/general/IPv6-Olympics-2008.htm|title=IPv6 and the 2008 Beijing Olympics|last=Das|first=Kaushik|year=2008|work=IPv6.com|access-date=15 August 2008|archive-date=1 August 2008|archive-url=https://web.archive.org/web/20080801051918/http://www.ipv6.com/articles/general/IPv6-Olympics-2008.htm|url-status=dead}}</ref>

By 2011, all major operating systems in use on personal computers and server systems had production-quality IPv6 implementations. Cellular telephone systems presented a large deployment field for Internet Protocol devices as mobile telephone service made the transition from [[3G]] to [[4G]] technologies, in which voice is provisioned as a [[voice over IP]] (VoIP) service that would leverage IPv6 enhancements. In 2009, the US cellular operator [[Verizon Communications|Verizon]] released technical specifications for devices to operate on its "next-generation" networks.<ref name="verizon">{{cite web|first=Derek|last=Morr|title=Verizon Mandates IPv6 Support for Next-Gen Cell Phones|url=http://www.circleid.com/posts/20090609_verizon_mandates_ipv6_support_for_next_gen_cell_phones/|publisher=CircleID|date=2009-06-09}}</ref> The specification mandated IPv6 operation according to the ''3GPP Release 8 Specifications (March 2009)'', and deprecated IPv4 as an optional capability.<ref name="verizon"/>

The deployment of IPv6 in the [[Internet backbone]] continued. In 2018 only 25.3% of the about 54,000 autonomous systems advertised both IPv4 and IPv6 prefixes in the global [[Border Gateway Protocol]] (BGP) routing database. A further 243 networks advertised only an IPv6 prefix. Internet backbone transit networks offering IPv6 support existed in every country globally, except in parts of [[Africa]], the [[Middle East]] and China.<ref name="IS 2018">{{cite web|title=State of IPv6 Deployment 2018|url=https://www.internetsociety.org/wp-content/uploads/2018/06/2018-ISOC-Report-IPv6-Deployment.pdf|website=InternetSociety.org|publisher=[[Internet Society]]|access-date=19 January 2022}}</ref>{{Rp|6}} By mid-2018 some major European [[broadband]] ISPs had deployed IPv6 for the majority of their customers. [[Sky UK]] provided over 86% of its customers with IPv6, [[Deutsche Telekom]] had 56% deployment of IPv6, [[XS4ALL]] in the Netherlands had 73% deployment and in Belgium the broadband ISPs [[VOO]] and [[Telenet]] had 73% and 63% IPv6 deployment respectively.<ref name="IS 2018"/>{{Rp|7}} In the United States the broadband ISP [[Xfinity]] had an IPv6 deployment of about 66%. In 2018 Xfinity reported an estimated 36.1 million IPv6 users, while [[AT&T]] reported 22.3 million IPv6 users.<ref name="IS 2018"/>{{Rp|7–8}}

==Peering issues==
There is a peering dispute going on between [[Hurricane Electric]] and [[Cogent Communications]] on IPv6, with the two network providers refusing to peer.<ref>{{cite web |title=The case of Hurricane Electric And Cogent |url=https://bgp.tools/kb/partitions |website=BGP.tools |access-date=10 September 2024}}</ref>

==See also==
{{Portal|Internet}}
*[[China Next Generation Internet]]
*[[Comparison of IPv6 support in operating systems]]
*[[Comparison of IPv6 support in common applications]]
*[[DoD IPv6 product certification]]
*[[OCCAID]]
*[[University of New Hampshire InterOperability Laboratory]]

==References==
{{Reflist}}

==External links==
{{Wikidata property | P3793 }}
{{Wikiversity|IPv6}}
{{Wiktionary|IPv6}}
*[https://www.haifux.org/lectures/187 IPv6 in the Linux Kernel] by Rami Rosen
*[https://www.google.com/intl/en/ipv6/ An Introduction and Statistics about IPv6] by Google
*[https://datatracker.ietf.org/doc/html/rfc8200 The standard document ratifying IPv6] – RFC 8200 document ratifying IPv6 as an Internet Standard

{{IPv6}}
{{Authority control}}

[[Category:IPv6| ]]
[[Category:Internet properties established in 1996]]
[[Category:Internet layer protocols]]
[[Category:Network layer protocols]]