Host Identifiers

Host identifiers used in messaging are 16 bytes long. Typically the host IP address is used as the host identifier. A 16-byte container is defined to accommodate the IPv6 protocol.

Advice to implementors.The following text highlights IPv6/IPv4 addressing issues. It is taken from RFC 2373 by R.Hinden & S.Deering:

The IPv6 transition mechanisms include a technique for hosts and routers to dynamically tunnel IPv6 packets over IPv4 routing infrastructure. IPv6 nodes that utilize this technique are assigned special IPv6 unicast addresses that carry an IPv4 address in the low-order 32-bits. This type of address is termed an ``IPv4-compatible IPv6 address'' and has the format:

|               80 bits                 | 16 bits |     32 bits    |
+---------------------------------------+---------+----------------+
|0000000000000..................00000000|000...000|  IPv4 address  |
+---------------------------------------+---------+----------------+

A second type of IPv6 address which holds an embedded IPv4 address is also defined. This address is used to represent the address of IPv4-only nodes (those that do not support IPv6) as IPv6 addresses. This type of address is termed ``IPv4-mapped IPv6 address'' and has the format:

|               80 bits                 | 16 bits |     32 bits    |
+---------------------------------------+---------+----------------+
|0000000000000..................00000000|111...111|  IPv4 address  |
+---------------------------------------+---------+----------------+

(End of advice to implementors.)