AAAA Record
IPv6 Address Record (Quad-A)
The IPv6 counterpart of the A record: it maps a hostname to a 128-bit IPv6 address. Publishing one alongside an A record makes a domain dual-stack, reachable over both IPv4 and IPv6.
An AAAA record, spoken as "quad-A," does for IPv6 exactly what the A record does for IPv4: it tells resolvers which address a hostname lives at. The only difference is the size of the answer. An IPv4 address is 32 bits; an IPv6 address is 128 bits — four times larger — which is why the record carries four A's in its name.
What an AAAA record looks like
; name TTL class type value
example.com. 3600 IN AAAA 2606:2800:220:1:248:1893:25c8:1946
www.example.com. 3600 IN AAAA 2606:2800:220:1:248:1893:25c8:1946The value is a literal IPv6 address in colon-hexadecimal notation. Like the A record, an AAAA record points directly at an address — never at another name — and it is legal at the zone apex.
Dual-stack: A and AAAA together
Most well-configured domains publish both an A and an AAAA record for the same name. This is called being dual-stack. An IPv6-capable visitor can use the AAAA record; an IPv4-only visitor uses the A record; nobody is left out.
| Records published | IPv4-only client | IPv6-capable client |
|---|---|---|
| A only | Connects over IPv4 | Connects over IPv4 |
| AAAA only | Cannot connect | Connects over IPv6 |
| A + AAAA (dual-stack) | Connects over IPv4 | Prefers IPv6, falls back to IPv4 |
Happy Eyeballs: how clients choose
When a name has both records, the client does not simply commit to IPv6 and hope it works. Modern operating systems and browsers use an algorithm called Happy Eyeballs (RFC 8305): they start an IPv6 and an IPv4 connection attempt almost in parallel, give IPv6 a small head start, and use whichever completes its handshake first. This means adding an AAAA record speeds up IPv6 users while costing IPv4-only users nothing — if IPv6 is slow or broken, the client quietly uses IPv4 instead.
When a bad AAAA record causes problems
The classic failure is publishing an AAAA record that points at an address the server cannot actually be reached on — for example, an IPv6 address configured in DNS but blocked by a firewall or never routed. On older clients without Happy Eyeballs, the browser tries IPv6 first, waits for it to time out, and only then falls back to IPv4, producing a multi-second delay on every connection. The fix is either to make the IPv6 path genuinely work or to remove the AAAA record until it does. Never publish an AAAA record "to be safe" if IPv6 is not truly reachable.
Looking up an AAAA record
# macOS / Linux
dig example.com AAAA +short
# Windows
nslookup -type=AAAA example.comIf the command returns one or more colon-hexadecimal addresses, the domain is reachable over IPv6. If it returns nothing while dig example.com A does return a value, the domain is IPv4-only — common but increasingly worth changing as IPv6 adoption grows.