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IPv6 applications compatibility with IPv4 applications
Socket applications written with AF_INET6 address family allow Internet Protocol version 6 (IPv6) applications to work with Internet Protocol version 4 (IPv4) applications (those applications that use AF_INET address family). This feature allows socket programmers to use an IPv4-mapped IPv6 address format. This address format represents the IPv4 address of an IPv4 node to be represented as an IPv6 address. The IPv4 address is encoded into the low-order 32 bits of the IPv6 address, and the high-order 96 bits hold the fixed prefix 0:0:0:0:0:FFFF. For example, an IPv4-mapped address can look like this:
::FFFF:192.1.1.1
These addresses can be generated automatically by the getaddrinfo() API, when the specified host has only IPv4 addresses.