In RFC 1886, a new DNS resource record type, AAAA (also known as "quad A"), is used for resolving a fully qualified domain name to an IPv6 address. AAAA records use the DNS record type of 28. AAAA records are comparable to the host address (A) resource records used for IPv4 name resolution. The resource record type is named AAAA because 128-bit IPv6 addresses are four times longer than 32-bit IPv4 addresses.
The AAAA resource record in a typical DNS database file has the following structure:
Name IN AAAA Address
Where Name is the fully qualified domain name and Address is the IPv6 address associated with the name. The following is an example of an AAAA resource record:
host1.microsoft.com IN AAAA FEC0::1:2AA:FF:FE3F:2A1C
To receive IPv6 address resolution data in the DNS query answer sections of the DNS query response, a host must specify either an AAAA query (by setting the Question Type field in a DNS query question entry to 0x1C [28 in decimal]) or a general query (by setting the Question Type field in a DNS query question entry to 0xFF [255 in decimal]).
RFC 1886 also describes the IP6.INT domain created for IPv6 reverse queries. Also called pointer queries, reverse queries determine a host name based on the address. To create the namespace for reverse queries, each hexadecimal digit in the fully expressed 32-digit IPv6 address becomes a separate level in the reverse domain hierarchy in inverse order.
For example, the reverse lookup domain name for the address FEC0::1:2AA:FF:FE3F:2A1C (fully expressed as FEC0:0000:0000:0001:02AA:00FF:FE3F:2A1C) is C.1.A.2.F.3.E.F.F.F.0.0.A.A.126.96.36.199.0.0.0.0.0.0.0.0.0.0.0.C.E.F.IP6.INT.
An example pointer (PTR) record is the following (folded for readability):
C.1.A.2.F.3.E.F.F.F.0.0.A.A.188.8.131.52.0.0.0.0.0.0.0.0.0.0.0.C.E.F.IP6.INT. IN PTR host1.microsoft.com
The DNS support defined in RFC 1886 is a simple way to both map host names to IPv6 addresses and provide reverse name resolution. It is a direct translation of IPv4 name and reverse name resolution techniques to IPv6.
DNS support for IPv6 is also described in RFC 2874, "DNS Extensions to Support IPv6 Address Aggregation and Renumbering." However, elements of this RFC are being designated by the IETF as experimental and are not described in this book.