What is reverse DNS?
Reverse DNS (rDNS) turns an IP address into a name. Normal DNS looks up an address for a name such as example.com. Reverse DNS goes the other way and answers: what name belongs to this address?
How it works
The owner of an address range publishes a PTR record, which links an address to a hostname. To look one up, the address is reversed and a special suffix is added. For example, 203.0.113.25 becomes 25.113.0.203.in-addr.arpa. IPv6 uses ip6.arpa in the same way, with each digit reversed.
What a result tells you
A hostname often hints at the owner or purpose, such as a mail server, a hosting machine or a customer connection. Many home connections have a generic automatic name, and some addresses have no reverse record at all.
Why it matters for email
Mail servers often check that the sending address has a reverse record that matches the name the server gives. If it does not, the message is more likely to be treated as spam. Anyone running a mail server needs to ask their provider to set the PTR record, because only the owner of the range can.
Limits to remember
The name is whatever the address owner chose to publish, so it is a clue, not proof. Forward and reverse records should match for a trustworthy server, which is called forward-confirmed reverse DNS.
Common questions
How do I look up reverse DNS?
Enter the address in the IP Lookup tool. It shows the hostname when one is published.
Why does my address have no reverse DNS?
The owner of the range has not published one. That is common for home connections.
Can I change my reverse DNS?
Only through whoever owns the address range, normally your internet or hosting provider.