What is an A record?
An A (Address) record is the DNS entry that maps a hostname such as example.com to an IPv4 address like 93.184.216.34. Its IPv6 counterpart, the AAAA record, maps the same hostname to a 128-bit IPv6 address. Together they are the terminal answer in almost every DNS lookup: aliases and CNAMEs ultimately resolve down to an A or AAAA record before a connection is made.
For email, A and AAAA records matter more than they first appear. Reverse DNS (PTR) records are expected to match the forward A record of a sending host, MTA-STS and autodiscover endpoints must resolve to a live address, and receivers routinely reject or throttle mail from hosts whose forward and reverse DNS do not line up. The record type is defined in RFC 1035.
Why check your A records?
1Confirm forward-confirmed reverse DNS
Many mailbox providers require a sending IP whose PTR points back to a matching A record — verify both sides agree
2Catch a stale or wrong IP
After a server migration a forgotten A record can silently route traffic to a decommissioned host
3Validate mail and web endpoints
MX targets, MTA-STS policy hosts, and tracking domains all need a resolvable A or AAAA record to work
4Verify IPv6 readiness
A missing AAAA record can cause slow or failed delivery to IPv6-first receivers such as Gmail
How A record resolution works — step by step
A client asks a resolver for the address of a hostname, for example mail.example.com.
The resolver walks the DNS hierarchy from the root to the authoritative nameserver for the domain.
The authoritative server returns the A record (IPv4) and, if present, the AAAA record (IPv6) for that hostname.
The resolver caches each answer for the length of its TTL and hands the IP back to the client.
The client opens a connection to the returned address — a web request, an SMTP session, or an API call.
A record syntax
example.com. 300 IN A 93.184.216.34
example.com. 300 IN AAAA 2606:2800:220:1:248:1893:25c8:1946Field breakdown
host— the name being resolvedTTL— cache lifetime in secondsIN— Internet record classA— maps to an IPv4 addressAAAA— maps to an IPv6 addressCommon A record failures and fixes
Missing AAAA record
IPv6-first receivers may deprioritise a host with no AAAA record — publish one that matches your A record
Forward/reverse mismatch
If the PTR record and A record disagree, receivers can reject mail — align both to the same IP
Pointing at a dead host
An A record left over from an old server sends users and mail nowhere — update it after every migration
Low TTL churn
An overly short TTL multiplies lookups and slows resolution — use a moderate value once the IP is stable
Frequently Asked Questions
An A (Address) record is a DNS entry that maps a hostname to an IPv4 address, while the AAAA record maps it to an IPv6 address. They are the final answer in most DNS lookups: when you visit a site or a server delivers mail, resolution ends at an A or AAAA record that returns the IP to connect to. Almost everything else — CNAMEs, MX targets, policy hosts — ultimately points at one.
An A record maps a hostname to a 32-bit IPv4 address (like 93.184.216.34), and an AAAA record maps the same hostname to a 128-bit IPv6 address (like 2606:2800:220:1:248:1893:25c8:1946). Many domains publish both so clients can reach them over either protocol. Some receivers, including Gmail, prefer IPv6, so a missing AAAA record can affect how your mail is handled.
Sending mail servers are expected to have forward-confirmed reverse DNS: the sending IP’s PTR record should resolve to a hostname whose A record points back to that same IP. When forward and reverse DNS disagree, many mailbox providers throttle or reject the mail. A records also back MX targets and MTA-STS policy hosts, so a broken one can quietly break delivery.
The most common causes are a record that was never published, a typo in the hostname, a record added to the wrong zone, or recent changes that have not propagated. DNS changes can take up to 48 hours to fully propagate, though most providers update within minutes. Confirm the record exists in your DNS panel and recheck after a short wait.
Yes. Publishing several A records for one hostname is a simple form of round-robin load balancing — resolvers hand out the addresses in rotation. Our checker lists every IPv4 and IPv6 address returned so you can confirm all of them point to healthy servers.