Using IPv4-Compatible Address Formats

In many instances, you can represent a 32-bit IPv4 address as a 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 enables you to experiment with different IPv6 deployments because you can use automatic tunneling to cross IPv4–only routers. However, you cannot configure these addresses by 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. An application can have a common address format for both IPv6 addresses and IPv4 addresses. The common address format can represent an IPv4 address as a 128-bit mapped address. However, IPv4–to-IPv6 protocol translators also allow these addresses to be used.