The policy table for IPv6 default address selection is a longest-matching prefix lookup table, much like a routing table. You can configure this table to suit your environment.

Given an address, a lookup in the policy table produces two values: a precedence value for the address and a label for the address. In the table, IPv4 addresses are represented as IPv4-mapped IPv6 addresses. The default policy table for IPv6 default address selection contains the following values.

In the table, the prefix values specify the address prefix that is used to select the policy table entry that best matches a source or destination address; the precedence values specify how destination addresses are sorted; and the label values specify whether a given source address prefix is preferred for use with a given destination address prefix.

This default configuration produces the following results:

• Native source addresses are preferred for use with native destination addresses
• 6to4 source addresses are preferred for use with 6to4 destination addresses
• IPv4-compatible IPv6 source addresses are preferred for use with IPv4-compatible IPv6 destination addresses
  Guideline: IPv4-compatible IPv6 addresses are deprecated by RFC 4291, but are shown here because they are part of the default policy table that RFC 3484 defines.
• Communication using IPv6 addresses is preferred to communication using IPv4 addresses, if matching source addresses are available

You can use the DEFADDRTABLE TCP/IP profile statement to configure the policy table for IPv6 default address selection to better suit your environment. For example, you can specify that IPv4 addresses should be preferred over IPv6 addresses.