DNS is used in the IPv6 world to do name-to-address mappings and vice versa. Mixed IPv4/IPv6 environments need multiple host entries in DNS. A host communicating with both versions of TCP/IP needs at least two entries in DNS—one with its IPv4 address and the other with its IPv6 address. Two new DNS record types have been defined for IPv6 hosts. RFC 1886 defines the AAAA type record (called quad-A), and RFC 2874 defines the A6 type record, which is designed to make renumbering of networks and TLA changes easier to administer.

For information about using DNS in combination with NAT-PT (the “PT” stands for Protocol Translation), which is described in Chapter 10, refer to RFC 2766.

AAAA Records (RFC 1886)

RFC 1886 describes DNS extensions for IPv6 implementations based on AAAA records. This record type can store a 128-bit IPv6 address, and the DNS value for this type of record is 28 (decimal). A host that has more than one IPv6 address has an AAAA record for each address. The corresponding reverse lookup domain is IP6.INT. The reverse lookup records are PTR records of type 12.

An AAAA type record can look like this (RFC 1886):

moon.universe.com   IN   AAAA   4321:0:1:2:3:4:567:89ab

For reverse lookups, each subdomain level under IP6.INT represents 4 bits of the 128-bit address. The least significant bit appears at the far left of the domain name. Omitting leading zeros is not allowed in this case. So the PTR record for the previous example looks like this:

b.a. ...

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