Reverse DNS

The Reverse DNS tool turns an IP address into a hostname by querying the PTR (pointer) record for that IP. PTR records live in the `in-addr.arpa` (IPv4) or `ip6.arpa` (IPv6) reverse zones, and are usually configured by the network operator that owns the IP — not by the end user or hosting customer. Reverse DNS is critical for outbound mail deliverability: most mail servers require the sending IP to have a valid PTR that matches its forward A record before they accept mail.

System administrators run reverse DNS lookups to investigate suspicious traffic, verify that a mail server's PTR aligns with its EHLO/HELO banner, audit shared-hosting IPs, and identify the ISP or cloud provider behind an IP. Security teams use PTR lookups during incident response — a PTR ending in `.amazonaws.com` versus `.compute.hetzner.cloud` versus `.somewhere-residential.net` tells you very different stories about what an IP is likely doing.

Not every IP has a PTR record. Cloud providers assign generic PTRs by default (`ec2-1-2-3-4.compute-1.amazonaws.com`) which you can usually override on request. Consumer ISPs assign residential-style PTRs. IPs with no PTR at all often signal poorly managed infrastructure or block-listed ranges.