Reverse DNS (PTR) Lookup

Reverse DNS (PTR) lookup finds the hostname associated with an IP address. It works in the opposite direction of a forward lookup (domain → IP), letting you identify an IP seen in server logs or check that a mail server's PTR is set up correctly. Both IPv4 and IPv6 are supported. For the opposite direction — a forward domain→IP lookup — use DNS Record Lookup.

The lookup reverses the IP and queries the PTR record under the in-addr.arpa (IPv4) or ip6.arpa (IPv6) domain via a public resolver over DNS over HTTPS. Just enter an IP. Your input is used only for the PTR query and is not stored externally.

What a PTR record is

A PTR (Pointer) record maps an IP address back to a hostname. For example, 8.8.8.8 becomes the query name 8.8.8.8.in-addr.arpa and resolves to a hostname such as dns.google. PTR is managed separately from forward A/AAAA records, so the two may not always match.

Where it is used

• Mail servers: many receiving servers treat a sending IP without a PTR as spam. A PTR is essentially required for reliable delivery.
• Log analysis: infer which organization or ISP an IP in your access logs belongs to.
• Network checks: confirm forward and reverse mappings agree (FCrDNS).

When the result is empty

PTR records must be set by the IP owner (ISP or cloud provider) in the delegated reverse zone. Most home and dynamic IPs have no PTR or only a generic hostname. An empty result does not mean the IP is invalid.

How the query name is built (worked example)

The heart of a reverse lookup is turning the IP "inside out" into a PTR name under a special domain. IPv4 and IPv6 follow different rules.

• IPv4 — reverse the four octets and append .in-addr.arpa.1.2.3.4 → 4.3.2.1.in-addr.arpa.
• IPv6 — fully expand the address to 32 hex digits, split it one nibble at a time in reverse order separated by dots, then append .ip6.arpa. 2001:4860:4860::8888expands to 2001:4860:4860:0000:0000:0000:0000:8888, and the query name becomes8.8.8.8.0.…(elided)…0.6.8.4.0.6.8.4.1.0.0.2.ip6.arpa. That is why every IPv6 PTR name ends with exactly 74 dots (32 nibbles + 2).

PTRs of well-known IPs (reference)

What the PTR resolves to matters less than whether resolving that hostname forward returns the same IP (FCrDNS). To confirm the forward direction, look up the A/AAAA of the PTR hostname with DNS Record Lookup.

Common pitfall

• Thinking you can "fix" a PTR by adding an A record: adding an A record in your forward DNS does not create a PTR. PTRs live only in the reverse delegated zone for that IP — typically your ISP or cloud console — and cannot be set from your own domain's zone file.
• Expecting one PTR per virtual host: an IP serving 100 domains should still have one PTR, not 100. One PTR that lines up with your SMTP banner and certificate hostname is what matters.
• Propagation confusion: right after an ISP sets a PTR, cache and delegation TTLs can leave the result empty for hours.