GlossaryDNS
A record
Definition
An A record ties a name to an IPv4 address: it tells the browser which machine to fetch the site from. Its IPv6 equivalent is called AAAA, and both can live on the same name.
The line you edit on launch day
Changing hosts means replacing the old server's address with the new one in this record. A domain usually carries two, one for the bare name and one for the “www” subdomain, unless the second is an alias pointing at the first.
For an agency, the useful question is not which address but since when. An A record still pointing at a server you stopped invoicing is a site that shows something under your client's name: the host's parking page, an old version of the site, or whatever a stranger put there the day that address was reassigned.
Three lines for one site
The bare name, the “www” subdomain, then the same machine over IPv6.
example.com. 3600 IN A 203.0.113.10
www.example.com. 3600 IN A 203.0.113.10
example.com. 3600 IN AAAA 2001:db8::10The first two lines are independent of each other: changing the top one changes nothing about the second. The third matters more than it looks, because a visitor on an IPv6 connection will prefer it — leave it behind and they are the only ones still seeing the old site.
The usual mistake
Updating the bare name, forgetting “www”, and getting a message the next day saying the site works “except for some people”. Those people are the ones who type the prefix, or have it bookmarked.
How to avoid it: Treat the bare name and “www” as a pair: check both before you announce a launch, and check the AAAA too if there is one. Three thirty-second lookups against a phone call from the client tomorrow.
Source: The A record is defined in RFC 1035 and AAAA in RFC 3596. The addresses used here are the ones RFC 5737 and RFC 3849 reserve for documentation.
Related terms
Where to go next
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