How to use Reverse DNS Lookup
The Reverse DNS tool performs a PTR lookup, turning an IP address back into the hostname its owner has published for it. Forward DNS maps names to addresses; reverse DNS does the opposite and is widely used by mail servers to sanity-check senders, by logging systems to label traffic, and by administrators verifying that an IP is configured correctly. A missing or generic PTR record is a common reason outbound mail is rejected, so checking the reverse record of your sending IP is an important deliverability step.
- Enter the IPv4 or IPv6 address you want to resolve.
- Click Lookup to query the PTR record in the reverse DNS zone.
- Read the hostname the address resolves back to, if any.
- Compare it against the forward record for a matching pair.
- Ask your ISP or host to set a proper PTR if one is missing.
How reverse DNS is structured
Reverse lookups use a special zone: IPv4 addresses are queried under in-addr.arpa with the octets reversed, while IPv6 uses ip6.arpa with nibble-reversed hex digits. Crucially, the PTR record is controlled by whoever owns the IP block — usually your hosting provider or ISP — not by the domain owner. That means you often cannot set your own PTR directly and must request it from the provider. This ownership split is why reverse DNS is frequently overlooked even when forward DNS is perfect.
Reverse DNS and email deliverability
Receiving mail servers routinely perform a reverse lookup on the connecting IP and check that it resolves to a sensible hostname, and ideally that the hostname resolves forward back to the same IP — a matching pair known as full-circle or FCrDNS. A sending IP with no PTR, or one pointing at a generic dynamic-pool name, looks suspicious and is a frequent cause of mail being throttled or rejected. Setting a proper PTR that matches your mail hostname is one of the simplest, highest-impact deliverability fixes.
| Address type | Reverse zone |
|---|---|
| IPv4 | in-addr.arpa (octets reversed) |
| IPv6 | ip6.arpa (nibbles reversed) |
Glossary
- PTR record
- A DNS record mapping an IP address back to a hostname.
- Reverse DNS
- Resolving an IP address to a name, the inverse of normal forward DNS.
- in-addr.arpa
- The special DNS zone used for IPv4 reverse lookups.
- FCrDNS
- Full-circle reverse DNS — forward and reverse records that agree.
- Nibble
- A four-bit hex digit, the unit IPv6 reverse zones are built from.
Related reading
Frequently Asked Questions
Why use Reverse DNS Lookup?
- Resolve an IP address to the PTR hostname its owner has published in DNS
- Confirm a sending IP has a proper reverse record before using it for outbound mail
- Check that forward and reverse records form a matching FCrDNS pair
- Identify the hostname behind a suspicious IP address found in server logs
Common use cases
- Verify a mail server IP has a PTR record before going live to avoid delivery rejections
- Check that a new VPS has a correct reverse record set by the hosting provider
- Confirm forward and reverse records agree after a server migration
- Identify an IP in access logs that belongs to a known scanning service
- Diagnose why a receiving mail server rejects connections from your IP address
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