Reverse IP lookup

Why this needs a different kind of source than every other tool here

DNS answers "what address does this domain point to." Nothing in DNS or RDAP records the reverse mapping - which domains point at a given address - because no protocol requires that to be published anywhere. The only way to answer it at all is from a dataset that has observed many domains resolving over time and recorded which addresses they used, which is inherently a third-party, best-effort source rather than something this server can derive from a protocol query of its own.

What this actually queries

A free, no-account passive-DNS API (HackerTarget) that returns hostnames it has previously observed resolving to the address given. It is rate-limited on their end - a limit reached shows as an honest error, never as an empty or fabricated result.

Reading a large, unrelated-looking list

An address shared by genuinely unrelated sites is completely normal, not a sign of anything wrong - most small and mid-sized sites sit on shared hosting or a CDN's IP space, alongside thousands of others with nothing in common but the infrastructure provider. A single dedicated IP with one clearly-related domain on it is the exception, not the rule, on the modern web.

What it is genuinely useful for

Spotting that a domain shares hosting with sites you did not expect (worth investigating for a security review), or roughly gauging how large a hosting provider's footprint on a given address block is.

Related tools

See IP address lookup for who the address itself is registered to.