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

An A record maps a hostname to a 32-bit IPv4 address; an AAAA record maps the same hostname to a 128-bit IPv6 address. They are separate record types and a domain can publish both. Clients with working IPv6 connectivity usually try the AAAA answer first and fall back to the A record when it does not connect.

Why do resolvers return different addresses for the same domain?

Usually because DNS propagation has not finished: each resolver serves its cached copy until the TTL expires, so a recent change is visible from some and not from others. Some providers also answer differently by region or by anycast location, in which case the disagreement is deliberate and permanent rather than a stage in a change.

How long does a DNS change take to take effect?

At most the TTL that was set on the old record, counted from the last time each resolver asked. If the previous record had a TTL of 3600 the change is complete within an hour; if it had 86400 it can take a day. Lowering the TTL before a planned change is what shortens the window.

Can a domain have an A record and an MX record at the same time?

Yes, and most do. The A record says where web traffic goes, the MX record says where mail goes, and they can point at entirely different hosts. The only interaction is the fallback rule: when a domain publishes no MX record, senders deliver to the address in its A record instead.

A record lookup: check IPv4 and IPv6 for a domain

What an A record is, and what AAAA adds

An A record maps a name to a 32-bit IPv4 address. An AAAA record does the same job for a 128-bit IPv6 address, and a domain may publish both; a client that has working IPv6 will generally try the AAAA answer first and fall back to IPv4 if the connection stalls.

A name may carry several A records at once. Resolvers hand them out in rotation, which spreads connections but is not failover: nothing removes an address from the answer because the host behind it stopped responding.

An address pair at the apex of example.com and a CNAME pointing the www name at it.

example.com.        300  IN  A      203.0.113.10
example.com.        300  IN  AAAA   2001:db8::10
www.example.com.    300  IN  CNAME  example.com.

Why two resolvers return different answers

DNS propagation is not a push. Nothing sends your change anywhere: each resolver keeps the previous answer until the TTL it was given expires, and only then asks again. A record with a TTL of 86400 can therefore be served in its old form for a full day after you edited it, from caches you have no access to.

SenderProbe asks every resolver it is configured with in parallel and compares the answers. Agreement means the change has finished. Disagreement is worth knowing about before you spend an afternoon debugging a mail flow that is only broken for the half of the internet still holding the old address.

What the A record lookup reports

Every A and AAAA record for the name, with its TTL.
Whether the domain publishes an IPv6 address at all, or IPv4 only.
Whether the resolvers agree, and which answer each one returned.
The PTR name behind each address, and whether it is forward-confirmed.
A domain that resolves to nothing, which is a different fault from one that resolves to the wrong place.

The apex, CNAMEs and the record you cannot publish

A CNAME says that a name is an alias for another name, and RFC 1034 forbids a name from carrying a CNAME alongside any other record. The apex of a domain always carries NS and SOA records, so it cannot carry a CNAME, which is why a hosting provider that wants a name is met with either an A record or a provider-specific alias record that is expanded at the nameserver instead.

This matters for mail because the apex is where the MX and SPF records live. An arrangement that tries to alias the apex tends to take those records with it.

What an A record has to do with email

More than it looks. A sender that finds no MX record falls back to the A record, so a domain that resolves at all will be offered mail whether or not it wants any. The a and mx mechanisms in an SPF record are evaluated by resolving addresses, so a change here changes who is authorised to send. And the address itself is what a receiving filter checks against blocklists before it reads a single word of the message.

Run this check on your own domain. A workspace takes a minute to create and runs every one of the eleven checks against the domain you name.

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Common questions

What is the difference between an A record and an AAAA record?: An A record maps a hostname to a 32-bit IPv4 address; an AAAA record maps the same hostname to a 128-bit IPv6 address. They are separate record types and a domain can publish both. Clients with working IPv6 connectivity usually try the AAAA answer first and fall back to the A record when it does not connect.
Why do resolvers return different addresses for the same domain?: Usually because DNS propagation has not finished: each resolver serves its cached copy until the TTL expires, so a recent change is visible from some and not from others. Some providers also answer differently by region or by anycast location, in which case the disagreement is deliberate and permanent rather than a stage in a change.
How long does a DNS change take to take effect?: At most the TTL that was set on the old record, counted from the last time each resolver asked. If the previous record had a TTL of 3600 the change is complete within an hour; if it had 86400 it can take a day. Lowering the TTL before a planned change is what shortens the window.
Can a domain have an A record and an MX record at the same time?: Yes, and most do. The A record says where web traffic goes, the MX record says where mail goes, and they can point at entirely different hosts. The only interaction is the fallback rule: when a domain publishes no MX record, senders deliver to the address in its A record instead.

Keep reading

Related checks, definitions and guides.

Resolve a domain and compare what each resolver returns