A and AAAA Record Lookup

What A and AAAA records are

An A record maps a hostname to an IPv4 address such as 192.0.2.10. An AAAA record does the same for IPv6, such as 2001:db8::10; the name comes from the address being four times the size of an IPv4 one. A name can have both, and most modern sites do. When it has both, clients try IPv6 and IPv4 almost in parallel and use whichever connects first (the Happy Eyeballs method, RFC 8305), so a broken IPv6 address usually costs a small delay rather than an outage.

How to read the result

The table lists every IPv4 and IPv6 address the name resolves to, with the TTL that tells resolvers how long to keep each answer. If the name is an alias, the result says so and names the target: the addresses then belong to the end of the CNAME chain, and changing them means changing the target, not your own record. Several addresses of one type are normal; resolvers rotate them and clients try the next one when the first does not answer.

IPv6: missing AAAA and broken AAAA

A name without an AAAA record still works for almost everyone, because IPv6-only clients usually reach IPv4 services through their provider's translation. Adding AAAA is worthwhile when your hosting supports IPv6, but only if the server really listens on that address. A stale AAAA left over from a previous host is the worse problem: clients that prefer IPv6 try it first, and some tools and older clients wait for it to time out before falling back. If you move a site, update or remove the AAAA record together with the A record.

Common problems

The apex (example.com) and www pointing at different servers after a migration, because only one of the two was updated. An old A record left next to the new one, so a share of visitors still reaches the old server. A private address such as 10.0.0.5 or 192.168.1.20 published in public DNS, which nobody outside your network can reach and which tells everyone a little about your internal layout. And an A record next to a CNAME at the same name, which DNS does not allow; most DNS panels refuse it, but zone files imported by hand sometimes contain it.

A records, CNAMEs and the zone apex

Use A and AAAA records for the apex of a domain: a CNAME cannot sit there, because the apex also carries the SOA and NS records. For subdomains you can choose. A CNAME to a hosting provider's hostname follows their address changes automatically; an A record is one lookup faster but has to be updated by hand whenever they renumber. Some DNS providers offer an ALIAS or flattened CNAME at the apex, which publishes A and AAAA records that track another name for you.

Primary sources

• RFC 1035: domain names, implementation and specification (A records)
• RFC 3596: DNS extensions to support IPv6 (AAAA records)
• RFC 8305: Happy Eyeballs version 2