Defined in RFC 1035, section 3.4.1:
domainname A 32-bit-ip-address
This defines the IP version 4 Internet address of the domain name. The IP address is written in the usual dotted quad format—for example, 192.168.55.2.
Defined in RFC 1886:
domainname A 128-bit-IPv6-address
IP version 6 uses 128-bit addresses, which is a lot more bits than in the version of IP currently in use, version 4. With IPv6, reverse lookups are performed in the ip6.int domain instead of in-addr.arpa. In addition, a dot exists between each nibble instead of between each octet, and the nibbles are in reverse order of course.
This record is not defined in any RFC and is not implemented in BIND. ...