What this tool checks
Every IP address on the public internet belongs to an Autonomous System — a network operated by a specific organization, whether that's a hosting company, an ISP, or a large enterprise running its own infrastructure. This tool resolves your domain's IP address, then looks up which Autonomous System announces that IP in the global routing table, via Team Cymru's free DNS-based lookup service.
Why this differs from a WHOIS lookup
A domain's WHOIS record tells you who registered the domain name — often a registrar's privacy proxy, telling you nothing useful. The ASN tells you who operates the server infrastructure actually serving the site, which is frequently a completely different organization (a cloud provider, a CDN, a specialized host) than whoever's name appears in WHOIS. For understanding what's actually behind a site — where it's physically hosted, whether it's behind a major CDN, what jurisdiction its infrastructure falls under — the ASN is often the more useful answer.
How the lookup works
Team Cymru publishes IP-to-ASN mapping data via a clever DNS trick: querying a specially-formatted hostname (the IP's octets reversed, followed by .origin.asn.cymru.com) as a TXT record returns the ASN and BGP prefix announcing that address. A second query against the ASN itself returns its human-readable name. Both are free, keyless, and don't touch any rate-limited HTTP API.