Autonomous system number (ASN)
A unique number that identifies an autonomous system — a collection of networks under a common administrative control and routing policy — on the internet. BGP uses ASNs to track which networks a route has passed through. The fundamental identity in the internet's routing.
Mechanic: every organization that runs its own BGP toward the internet (operators, large enterprises, content providers) gets an ASN assigned by an RIR (a regional internet registry like RIPE, ARIN). The ASN represents their network in BGP — it shows up in the AS-path of all routes they advertise. Format: originally 16-bit (0–65535), now extended to 32-bit because of running out of numbers. Private ASN ranges exist (like private IP) for internal use. Examples: Google = AS15169, Cloudflare = AS13335. If you want to see who owns an IP, you look up its ASN. Together with the IP prefix, the ASN is what defines "a network" on the internet. Related to BGP peering and Tier 1 network.