A and AAAA Records: Pointing a Domain to an IP Address

An A record (short for "address") maps a hostname to a single IPv4 address. IPv4 addresses are 32-bit numbers written as four dot-separated decimals — for example 198.51.100.10. Each number falls between 0 and 255.

When a resolver looks up example.com and finds an A record, it learns "to reach this name over IPv4, connect to this address." It is the most common record type in DNS, and for many years it was the only one most sites needed.

An AAAA record (pronounced "quad-A") does the same job for IPv6. IPv6 addresses are 128-bit numbers written as groups of hexadecimal digits separated by colons — for example 2001:db8::1. The name "AAAA" comes from the fact that an IPv6 address is four times the size of an IPv4 address.

IPv6 exists because the world ran out of spare IPv4 addresses. A growing share of clients — especially mobile networks — prefer IPv6 when it is available, so publishing an AAAA record alongside your A record lets those users connect natively instead of through translation. See IPv4 vs IPv6 for the bigger picture.

Whether you are editing an A or an AAAA record, you fill in the same handful of fields:

• Name (host): the hostname the record applies to, such as the apex example.com or a subdomain like www.example.com. Many DNS editors show this as @ for the apex.
• Type: A for IPv4 or AAAA for IPv6.
• TTL: the time-to-live, in seconds — how long resolvers may cache this answer before checking again. See TTL and propagation.
• Value (data): the address itself — a dotted-decimal IPv4 for an A record, or a colon-separated IPv6 for an AAAA record.

That is the whole record. The only real difference between A and AAAA is the kind of address that goes in the value field.

Here is a small, complete record set for example.com that serves both IPv4 and IPv6 at the apex and on www:

A visitor whose network supports IPv6 will typically use the AAAA record and connect to 2001:db8::10; a visitor on IPv4-only will use the A record and connect to 198.51.100.10. Both reach the same server, so the experience is identical.

A single name can have more than one A record (and more than one AAAA record). When it does, the DNS server hands back the full list, and the client picks one to connect to. Rotating which address appears first is called round-robin DNS, and it is a simple way to spread traffic across several servers.

It is worth being precise about what this does and does not give you. Round-robin is not true load balancing: DNS has no idea which server is busy, healthy, or even reachable. The client chooses (modern clients use "happy eyeballs" to race IPv4 and IPv6 and fall back quickly), so a dead server in the list can still cause slow or failed connections until you remove it. For real health-aware distribution you want a load balancer behind a single record, not many records.

The apex (or root) of your domain is the bare name, example.com, with no host in front. A subdomain is anything to the left, like www, blog, or api.

On a subdomain you can often point at a provider's hostname with a CNAME record instead of an IP. At the apex you usually cannot: standard DNS rules forbid a CNAME on a name that also has other records (and the apex always has SOA and NS records), so the apex generally needs real A and AAAA records with literal IP addresses. Some DNS providers offer a workaround called ALIAS or ANAME / "CNAME flattening" that resolves the target for you and answers with A/AAAA, but the record published at the apex is still an address record.

Publishing both an A and an AAAA record for the same name makes your site dual-stack: reachable over IPv4 and IPv6 at once. This is the recommended setup for almost every public service today.

Two things to keep in mind. First, an AAAA record is a promise that your server actually listens on that IPv6 address — if it does not, IPv6-preferring clients can stall before falling back to IPv4. Second, the A and AAAA records are independent; updating one does not touch the other, so it is easy to fix an IPv4 address and forget the matching IPv6 one. For more on the two protocols, see IPv4 vs IPv6.

The TTL on your A and AAAA records controls how long resolvers around the world cache the old address. If your TTL is 86400 (one day) and you change the IP, some visitors can keep hitting the old server for up to a day after the change goes live.

Before a planned move to a new IP, lower the TTL first — for example to 300 seconds (five minutes) — and wait for the old, longer TTL to expire so caches everywhere have picked up the short value. Then make the address change: it will propagate quickly, and you can raise the TTL back up once you have confirmed the new server is serving traffic. TTL and propagation covers the timing in detail.

• Pointing at the wrong IP. A transposed digit or an internal/old address sends visitors nowhere. Confirm the exact address with your host before you save.
• Forgetting the AAAA record. Adding only an A record leaves IPv6 users routed through translation (or failing on IPv6-only networks). If your server supports IPv6, publish the AAAA too.
• Updating only one of the pair. Because A and AAAA are separate records, changing the IPv4 address while leaving a stale IPv6 address sends half your traffic to a dead server.
• Stale records after a TTL you forgot to lower. A long TTL means the old answer lingers in caches well after you make a change.
• Mixing a CNAME with A/AAAA on the same name. A name with a CNAME cannot also have A or AAAA records; DNS treats that as invalid.

To see what a domain currently publishes, run a DNS lookup on who.is and read the A and AAAA rows — they show the live IPv4 and IPv6 addresses every resolver is handing out, along with each record's TTL.

If you want to go the other direction and find out which domains point at a given address, an IP lookup shows who an address is registered to, and a WHOIS lookup covers the registration details of the domain itself.