AAAA record
AAAAThe AAAA record is the IPv6 equivalent of an A record. The name is four As because an IPv6 address is four times the size of an IPv4 one.
What an AAAA record does
An AAAA record maps a hostname to a 128-bit IPv6 address, in the same way an A record maps to a 32-bit IPv4 address. The record type is defined in RFC 3596.
A name can publish A and AAAA records at the same time. They are independent records, and publishing one says nothing about the other.
How clients choose
A dual-stack client typically queries for both A and AAAA and decides which to use based on what its own network supports. Many clients implement Happy Eyeballs (RFC 8305), which starts connections over both and keeps whichever answers first.
That means publishing a broken AAAA record is worse than publishing none: clients will try IPv6, wait, and fall back, adding latency to every connection.
Common mistakes
Publishing an AAAA record for a server that is not actually reachable over IPv6 is the usual failure. The record resolves, the connection times out, and the user sees a slow site rather than an error.
Address formatting is the other one. IPv6 addresses are written in hexadecimal with colons and may be abbreviated with a double colon, which can only appear once in an address.