An Autonomous System Number (ASN) identifies a distinct network on the internet — typically an ISP, a hosting provider, or a large organization that controls its own block of IP addresses and makes its own routing decisions.
A network of networks
The internet isn't one network — it's thousands of independently operated networks, each called an autonomous system, connected to each other. Your home connection's traffic passes through your ISP's autonomous system before reaching whatever network hosts the site you're visiting. Each one is identified by a unique ASN, written like AS64500.
Why your IP has one
Every public IP address belongs to a block that's been allocated to a specific ASN. Looking up which ASN owns your IP is how you get the "ISP / Org" field shown on the home page — it's the organization responsible for the network your address sits in, not a person or exact location.
ASNs and routing
Autonomous systems exchange routing information with each other using a protocol called BGP (Border Gateway Protocol), essentially announcing "traffic for these IP ranges should come through me." That's the mechanism that lets a packet find its way across dozens of independently owned networks to reach the right destination.
Datacenter vs residential
ASN lookups often distinguish datacenter networks (cloud providers, hosting companies) from residential/ISP networks. This distinction matters for anti-fraud and anti-bot systems, which is part of why VPN and datacenter IPs sometimes get treated differently by websites than a typical home connection.