Find the hostname registered behind any IPv4 address via its PTR record — and see whether it’s forward-confirmed (FCrDNS), the consistency check mail servers use.
Reverse DNS resolves an IP address back to a hostname via its PTR record — the opposite of a normal lookup that turns a domain into an IP. Where forward DNS answers “what IP is example.com?”, reverse DNS answers “what hostname belongs to this IP?” This tool performs that lookup for any IPv4 address and shows the registered hostname, if one exists.
Mail servers rely on it heavily: a sending server with a missing or generic PTR record looks suspicious and its mail is often rejected or marked as spam. Network administrators use reverse DNS to identify the organisation behind traffic in their logs, and the strongest configuration — Forward-Confirmed Reverse DNS (FCrDNS) — requires the PTR and the hostname’s A record to point at each other. If you run a mail server, valid reverse DNS is essential for deliverability.
Many residential and mobile IPs have a generic ISP-assigned hostname (like host-203-0-113-45.isp.net) or no PTR record at all — that’s expected and doesn’t indicate a problem for ordinary browsing. A missing PTR only matters when the IP is sending email or running a service that depends on it.
Learn more: What is a PTR record? · Reverse DNS from the command line · Fix FCrDNS errors