What is an A record? (A and AAAA, explained)

The one-sentence version

An A record maps a name to a 32-bit IPv4 address. Its twin, AAAA, maps a name to a 128-bit IPv6 address. That is the whole job: take a hostname someone typed, and return the numeric address a machine actually connects to.

example.com.       3600   IN   A      203.0.113.10
example.com.       3600   IN   AAAA   2001:db8::10
www.example.com.   3600   IN   A      203.0.113.10

A versus AAAA

These are the same record for two versions of the internet protocol. A answers with an IPv4 address (the familiar dotted-decimal form); AAAA answers with an IPv6 address (the longer, colon-separated form). They are not alternatives you choose between: if a server has both an IPv4 and an IPv6 address, you publish both, and clients use whichever they can reach. A name with no AAAA simply is not reachable over IPv6, which is usually fine, until it is your mail server, as we will see below.

The apex, and why you cannot CNAME it

Your bare domain (the “apex” or root, example.com with nothing in front) is almost always an A record, often with an AAAA alongside it. A common instinct is to point the apex at a host using a CNAME instead, and DNS will not let you. The apex must carry your SOA and NS records, and a CNAME is not allowed to coexist with any other record at the same name. So the apex stays an A or AAAA record. (Some providers offer an “ALIAS” or “CNAME flattening” option that looks like a CNAME at the root but quietly resolves to an address record for you; we cover that on the CNAME page.)

How A records decide whether your mail is trusted

This is where a humble address record reaches into your deliverability. Strict receivers, Gmail chief among them, will not fully trust mail from a sending IP unless it passes forward-confirmed reverse DNS (FCrDNS), a two-way check:

• The sending server's hostname has an A (or AAAA) record pointing to its IP address.
• That IP has a matching PTR record pointing back to the same hostname.

Both lookups have to agree. If the forward A record and the reverse PTR do not match, Google's and Yahoo's sender requirements treat the IP as untrustworthy and your mail is penalized or rejected. There is a sharper edge for IPv6: Gmail calls it out directly, returning a 550-5.7.1 bounce noting mail that “does not meet IPv6 sending guidelines regarding PTR records and authentication.” In practice, sending over IPv6 without a matching AAAA and PTR is a frequent cause of sudden Gmail rejections.

The MX connection

Your MX record never contains an IP address. It names a hostname, and that hostname is resolved through its A or AAAA record. So a missing or wrong address record on your mail server's hostname breaks inbound mail just as surely as a missing MX would: senders find the MX, look up the exchange, and get nothing to connect to.

Where A records touch your outbound authentication

SPF can authorize the addresses of a hostname directly, with the a mechanism. A record like v=spf1 a:mail.example.com -all says “treat whatever IPs mail.example.com resolves to as authorized senders.” It is the one place an A record feeds your outbound trust, and a reason an address-record change can quietly shift your SPF result.

How to check yours

• dig A example.com +short and dig AAAA example.com +short return the addresses your name resolves to.
• For mail, confirm your sending hostname resolves and its reverse matches: compare dig A mail.example.com +short against dig -x 203.0.113.10 +short. They should describe the same hostname.

Common questions

What is the difference between an A record and an AAAA record?

Only the IP version. A returns an IPv4 address, AAAA returns an IPv6 address. They do the same job, and a dual-stack server publishes both.

Can I use a CNAME instead of an A record at my domain root?

No. The root (apex) must hold SOA and NS records, and a CNAME cannot coexist with them. Use an A or AAAA record, or your provider's ALIAS / CNAME-flattening feature, which resolves to an address record behind the scenes.

Do I need an AAAA record to send email?

Only if you actually send over IPv6. If you do, the IPv6 address needs the same forward-and-reverse DNS match (a matching AAAA and PTR) that IPv4 needs, and Gmail enforces it strictly. If you send only over IPv4, you do not need an AAAA on your mail host.