§ 01
Definition
Networks that route traffic independently are called autonomous systems. Regional registries, RIPE NCC for Europe and the Middle East, assign each one a number, and it shows up in IP lookup tools next to the network's name.
§ 02
How it works
Websites look up the ASN behind your IP to see who operates the address. If the ASN belongs to a hosting provider rather than a consumer ISP, the visitor is probably on a VPN, proxy or server. Some VPN providers run their own ASN, which makes their servers easier to recognise.
§ 03
Why it matters
ASN data is one of the main reasons streaming services and banks can detect VPNs even when the IP itself is new. It is also why 'residential' IPs, which sit in consumer ISP ASNs, are harder to block.