The internet is not one network but tens of thousands of networks agreeing among themselves how traffic flows. Such a network, with its own routing policy, is an autonomous system, and it is given a unique number: the ASN.
What an AS does
An AS announces over BGP which address ranges it can reach. Other networks hear that announcement and thereby know where to send traffic for those ranges. The BGP prefix shown with an IP address is precisely that announcement.
Dutch examples
- AS1103 — SURF, the network of Dutch higher education and research
- AS8737 — KPN
- AS9143 — Ziggo
- AS1136 — KPN Internet Services
Why it is useful
The ASN is the most reliable way to say which network announces an address. Where geolocation is an estimate and WHOIS data can be stale, the routing table is an observation: this network is saying, right now, that it serves this range.