A record
An A record (Address record) is one of the most important entry types in the DNS: it maps a domain name to a specific server IPv4 address. When a browser looks up example.com, it is the A record that answers which IP number to find the machine serving the site at.
How an A record works
An A record lives in a domain's DNS zone and consists of a few fields:
- name — a subdomain or the @ symbol standing for the root domain,
- type — A,
- value — an IPv4 address as four numbers, e.g. 192.0.2.10,
- TTL — how long the answer may be held in cache.
A single name can have several A records pointing at different addresses — the resolver then returns them in rotation, giving simple load spreading (round-robin DNS). For IPv6 addressing there is the analogous AAAA record, and for pointing at an alias, the CNAME record.
The A record in practice
You set an A record whenever you point a domain at a hosting account or server: you enter the machine's IP address and the domain starts directing web traffic to it. During a migration to a new server, you change the A record's value to the new IP — which is why lowering the TTL beforehand is good practice, so the switch happens fast and without downtime. A records are edited in the DNS provider's or hosting control panel.
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How is an A record different from an AAAA record?
Both point to a server address, but an A record returns an IPv4 address (e.g. 192.0.2.10) while an AAAA record returns an IPv6 address. A domain can have both at once, making it reachable over the older and the newer version of the IP protocol.
How long does an A record change take to apply?
It depends on the TTL. Caching resolvers keep the old answer until it expires, so with a TTL of 3600 s the change spreads within an hour. Before a server migration, lower the TTL in advance so the switch is nearly instant.