Editing IPv6 (section)

==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-->