Find the hostname associated with an IP address — the reverse of a DNS lookup.
Reverse DNS maps an IP address back to a hostname — the opposite of what a normal DNS query does. It works through PTR records in the in-addr.arpa zone. When you see a hostname like server-1.hosting.com for 203.0.113.42, it is because the owner of that IP range configured a PTR record pointing to that name.
Not every IP has a reverse DNS record. Many hosting providers and ISPs set them up, but it is not mandatory. Email deliverability is the main reason PTR records matter: receiving mail servers often check that the sending IP has a reverse record that matches the sending domain. Without it, email is more likely to land in spam.
The lookup runs on the server using gethostbyaddr. The IP you look up is not stored.
A DNS record that maps an IP address to a hostname. It lives in the in-addr.arpa zone and is set up by whoever controls the IP range.
Because the owner of the IP range did not configure one. It is common for residential ISPs and small hosting providers.
Yes. Many mail servers check that the sending IP has a PTR record. Without one, your email is more likely to be flagged as spam.
The PHP function supports it, but this tool validates IPv4 format only.
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