IPv4-mapped IPv6 addresses

These addresses hold an embedded global IPv4 address. They are used to represent the addresses of IPv4 nodes as IPv6 addresses to applications that are enabled for IPv6 and are using AF_INET6 sockets. This allows IPv6-enabled applications to always deal with IP addresses in IPv6 format regardless of whether the TCP/IP communications are occurring over IPv4 or IPv6 networks. The dual-mode TCP/IP stack performs the transformation of the IPv4-mapped addresses to and from native IPv4 format. IPv4-mapped addresses have the following format:

Figure 1. IPv4-mapped IPv6 address

First 80 bits are all 0; next 16 bits are FFFF; last 32 bits are the IPv4 address.

For example:

::FFFF:129.144.52.38