Reverse DNS lookup
Shows the reverse name published for an IP address. It matters for mail servers: a missing or wrong PTR record is one of the common reasons outgoing mail is refused.
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What the reverse DNS lookup shows
A reverse DNS lookup shows the PTR record published for an IP address: the name the address claims to be. For IPv4 the question is put to the reversed octets under in-addr.arpa, so 8.8.8.8 is asked for as 8.8.8.8.in-addr.arpa, and the page shows that name; for IPv6 the question goes under ip6.arpa, one hexadecimal digit per label, and the page shows that name too. The question is put to Cloudflare's 1.1.1.1 from the IPVIEW server, through the toolkit's own DNS engine. A hostname is accepted too: it is resolved first and the reverse of that address is shown, with the domain you typed kept on the page.
PTR records are published not by the domain owner but by whoever holds the address block, usually the hosting provider or ISP. A private or reserved address is refused, since its reverse name is whatever the local network says; 127.0.0.2 is allowed as a self-test.
How to read the result
The page shows the address, the domain it was resolved from where there was one, the name queried in the arpa tree and the PTR names returned, or that no reverse name is published. Only a definite no such name counts as none; a timeout or server failure is reported as a failed lookup, not a missing record.
A PTR alone proves little. Receiving mail servers check forward-confirmed reverse DNS: the PTR name must itself resolve back to the same address, and a PTR of mail.example.com whose A record points elsewhere counts for nothing. The Full report performs that round trip for the web address and the first three mail addresses. A web address behind a CDN with a generic PTR, or none, is not a fault; a mail server in the same state is.
When it is useful
Outgoing mail is refused with a message about reverse DNS or a hostname mismatch, or lands in spam: see what name the sending address announces, then check with the A record tool that the name resolves back. Email blacklist check and Email security are the other two checks for the same server.
When reading a log, a Traceroute or an MTR report, a PTR name is the fastest way to learn whose router or server an address belongs to; WHOIS on the same address names the organisation holding the block.
Common questions
What is a PTR record?
The reverse of an A or AAAA record: it maps an address back to a name. It is published by the holder of the address block under in-addr.arpa or ip6.arpa, not by the owner of the domain.
Why does my mail server need reverse DNS?
Most receivers refuse or downgrade mail from an address with no PTR, or whose PTR does not resolve back to the same address. Have your provider set the PTR to your mail hostname and give that name an A record pointing at the address.
How do I set a PTR record?
In the control panel of the provider or ISP that assigned the address, or through their support. The zone belongs to them, not to your domain's nameservers.
Limits. The DNS, WHOIS and SSL tools allow up to 20 requests a minute per IP address. Ping, traceroute and MTR send real packets to their target, so they allow 6 a minute.
The port check runs only against your own IP address. That is deliberate: the question people actually have, "is this port reachable from outside?", is answered by it, while this server cannot be used to scan anyone else's network.
Private and reserved addresses. These are refused by the tools that really connect to their target: the SSL check, ping, traceroute, MTR and the port check. The DNS tools have no such limit, because they only ask a nameserver a question and never connect to the address itself.