Find the PTR (reverse DNS) hostname for your IP address — or any IPv4 / IPv6 address — and check forward-confirmed reverse DNS (FCrDNS), the test mail servers run.
10.x, 192.168.x, 127.x, …) have no public reverse DNS.Forward DNS turns a name like example.com into an IP address using A and AAAA records. Reverse DNS goes the other way: it turns an IP address back into a hostname using a PTR record. The two directions are configured independently and often by different parties — forward records by whoever runs the domain, reverse records by whoever was allocated the IP block (usually the ISP or hosting provider).
PTR records live in a special tree. For IPv4, the address is reversed octet by octet and placed under in-addr.arpa — 8.8.8.8 becomes 8.8.8.8.in-addr.arpa. For IPv6, each hex nibble is reversed and placed under ip6.arpa. Because those zones are delegated down the same hierarchy as the addresses, only the block holder can set the answer.
A PTR record on its own is easy to fake, so receiving systems apply a stricter test: take the hostname the PTR returns, resolve it forward, and check the original IP is in the result. If it matches, the reverse DNS is "forward-confirmed". This tool runs that check for you and shows the forward IPs it found.
crawl-66-249-66-1.googlebot.com instead of a bare number.traceroute and mtr come from PTR records and often encode the city and carrier.A PTR (pointer) record maps an IP address back to a hostname — the reverse of the A and AAAA records that map a name to an IP. It lives in the special in-addr.arpa zone for IPv4 and ip6.arpa for IPv6, and is controlled by whoever was allocated the address block, usually the ISP or hosting provider.
Usually not for browsing. It matters most for sending email: many mail servers reject or penalise a sending IP that has no PTR record, or whose PTR hostname does not resolve forward back to the same IP (a check called FCrDNS). If you run a mail server, ask your provider to set the PTR.
It performs one reverse DNS query and, for each hostname returned, a forward lookup to confirm it. Those queries go to the server's DNS resolver like any lookup; the IP address is not written to any log or database, and the result is cached briefly in memory.