🔁 Reverse DNS PTR Name Builder

Enter an IPv4 or IPv6 address to build the PTR lookup name used for reverse DNS — the in-addr.arpa or ip6.arpa name you would create a PTR record under.

How the name is built

Reverse DNS reads an address backwards, because DNS names go from most specific on the left to least specific on the right, while IP addresses run the other way. The construction happens in your browser; no lookup is performed.

IPv4 reverses the four octets and appends in-addr.arpa. IPv6 is expanded to its full 32 hex digits - :: filled back in, each group padded to four - then every single digit is reversed and separated by dots:

192.168.1.1   ->  1.1.168.192.in-addr.arpa
203.0.113.25  ->  25.113.0.203.in-addr.arpa
2001:db8::1   ->  1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa

Who can actually create the record

Not you, usually. The in-addr.arpa zone for an address block belongs to whoever was allocated that block, so a PTR record has to be added by your hosting provider, ISP or cloud console - never in your own domain's zone file. The forward record is yours; the reverse one is theirs. IPv4 blocks are normally delegated on /24 boundaries, and anything smaller needs the CNAME arrangement in RFC 2317.

Make the PTR agree with a forward record. If 25.113.0.203.in-addr.arpa points to mail.example.com, then mail.example.com must have an A record back to 203.0.113.25. Receiving mail servers check both directions and reject mismatches.

Frequently asked questions

Why can I not add a PTR record in my domain's DNS?

Because a PTR lives under in-addr.arpa, a zone controlled by the owner of the IP block, not by the owner of the domain. Ask your hosting provider or ISP to set it, or use the reverse DNS field in your cloud console.

Do I need reverse DNS to send email?

In practice yes. Large providers reject or heavily penalise mail from an address with no PTR, or with one that does not match a forward record pointing back at the same address.

Why is the IPv6 PTR name so long?

Because it uses one label per hex digit and an IPv6 address has 32 of them. That gives 32 dotted labels plus ip6.arpa, where IPv4 needs only four.