Autonomous System
Problem
BGP needs to reason about the Internet as independently administered networks.
Individual routers are too fine-grained.
Google may operate thousands of routers, but externally they belong to one routing administration.
Solution — Autonomous System
An AS is a group of networks managed by one organization with coordinated rules for exchanging traffic with other networks.
Each receives an ASN:
AS64500
AS64501
AS64502
BGP can then reason about routes through autonomous systems rather than every individual router.
Core idea: an AS is an independently administered routing domain on the Internet.