Reverse DNS Lookup
Look up the hostname associated with any IP address.
Enter an IPv4 or IPv6 address to find its hostname (PTR record).
What Is Reverse DNS?
Reverse DNS (rDNS) is the process of resolving an IP address back to a hostname. While standard "forward" DNS maps domain names to IP addresses, reverse DNS does the opposite — it maps IP addresses to domain names using PTR (pointer) records. PTR records are stored in a special zone under in-addr.arpa (for IPv4) or ip6.arpa (for IPv6), and they're managed by the organization that controls the IP address block, not the domain registrar.
Why Reverse DNS Matters
Reverse DNS is most commonly used in email delivery. Many mail servers reject or flag emails from IP addresses that lack a valid PTR record, because spammers often use IP addresses without rDNS configured. A properly configured PTR record that matches the mail server's hostname (forward-confirmed reverse DNS, or FCrDNS) significantly improves email deliverability. Network administrators also use rDNS for logging — replacing raw IP addresses with hostnames in access logs makes them much easier to read and audit.
Forward-Confirmed Reverse DNS (FCrDNS)
This tool performs an additional verification step called FCrDNS. After resolving the IP to a hostname via PTR lookup, it resolves that hostname forward (A/AAAA lookup) and checks if the original IP appears in the results. When both directions match, the rDNS is "forward-confirmed" — this is the gold standard for mail server configuration and provides strong evidence that the IP and hostname genuinely belong to the same entity.
Common Issues
If no PTR record is found, the IP's owner (usually the hosting provider or ISP) hasn't configured one. Many cloud providers require you to request rDNS setup through a support ticket. Mismatched rDNS — where the PTR hostname doesn't resolve back to the original IP — often indicates a configuration error that should be corrected, especially for mail servers.