A vs AAAA Records

An A record points a name to an IPv4 address; an AAAA record points the same name to an IPv6 address. They are not alternatives so much as the same record for two address families. The usual setup is to publish both, called dual stack, so clients on either protocol can connect.

An A record maps a hostname to a 32-bit IPv4 address, written as four numbers separated by dots:

IPv4 is the original addressing scheme of the internet and is still supported by essentially every network and client. Its limitation is the address space: roughly 4.3 billion addresses, which the internet exhausted years ago. That scarcity is the reason IPv6, and the AAAA record, exists.

An AAAA record maps a hostname to a 128-bit IPv6 address, written as groups of hexadecimal digits separated by colons:

The name comes from the address being four times the size of an IPv4 address (four As). IPv6 provides an effectively unlimited address space and is the long-term direction of the internet, with strong support across mobile and modern networks.

Dual stack simply means publishing an A record and an AAAA record for the same name, so your server is reachable over both protocols:

A client uses whichever protocol it supports. Modern browsers go further with Happy Eyeballs, attempting IPv4 and IPv6 in parallel and using whichever responds first. For dual stack to work, the server behind both addresses must actually serve the same content.

You can run a site with only an A record, because IPv4 still reaches everyone. But publishing AAAA records is increasingly worthwhile: a large share of mobile traffic is IPv6, native IPv6 connections avoid carrier translation gateways, and it future-proofs your domain. The main caveat is to add an AAAA record only if the server genuinely listens on that IPv6 address, since a stale AAAA record can cause hard-to-diagnose connection failures for IPv6 clients.

For background on how either record is looked up, see what is DNS.