Internet providers, hosting companies, CDNs, universities and large enterprises each run one or more autonomous systems. With BGP, each AS announces which IP prefixes it originates, and those announcements are how traffic finds its way between networks. ASNs are assigned by the five regional internet registries (RIPE NCC, ARIN, APNIC, LACNIC and AFRINIC) and can be 16-bit or 32-bit numbers.
Looking up the ASN that originates an IP address shows which network the address belongs to, which is usually the quickest way to identify the hosting company or CDN behind a website. OrbitProbe derives it from public routing data and the IP registries' RDAP services.
Two cautions when reading the result. The AS that announces a prefix is not always the company that runs the server: addresses can be leased, and a site behind a CDN or DDoS-protection service shows that service's ASN, not the origin host. And the country attached to a network is where the address block is registered, which is not necessarily where the server stands.
Example
198.51.100.0/24 originated by AS64500
(AS64496-AS64511 are reserved for documentation)