Using IPv4 Compatible Address Formats

In many cases you can represent a 32-bit IPv4 address as an 128-bit IPv6 address. The transition mechanism defines the following two formats.

• IPv4 compatible address

  000 ... 000

  IPv4 Address

• IPv4 mapped address

  000 ... 000

  0xffff

  IPv4 Address

The compatible format is used to represent an IPv6 node. This format enables you to configure an IPv6 node to use IPv6 without having a real IPv6 address. This address format lets you experiment with different IPv6 deployments because you can use automatic tunneling to cross IPv4-only routers. However, you cannot configure these addresses using the IPv6 stateless address autoconfiguration mechanism. This mechanism requires existing IPv4 mechanisms such as DHCPv4 or static configuration files.

The mapped address format is used to represent an IPv4 node. The only currently defined use of this address format is part of the socket API. It is convenient for an application to have a common address format for both IPv6 addresses and IPv4 addresses by representing an IPv4 address as a 128-bit mapped address. However, these addresses can also be used when there are IPv4 to IPv6 protocol translators.