Editing IPv6 (section)

===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-->