An address family identifier is used to identify individual network address schemes or numbering plans for network communication in contexts where the use of individual addresses might otherwise be ambiguous.
Address family identifiers were first defined in RFC 2453.
[1] Address family identifiers are used in communications protocols and APIs that support multiple network address schemes, including routing protocols such as BGP and RIPv2.
The list of address family identifiers is maintained by IANA.
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