自律システム (AS)

What Makes an AS Unique

Every Autonomous System is identified by an Autonomous System Number (ASN) — a globally unique integer assigned by Regional Internet Registries (RIRs) such as ARIN, RIPE NCC, and APNIC. ASNs were originally 16-bit (1-65535) but expanded to 32-bit to accommodate growth, enabling over four billion unique identifiers.

Large ISPs, cloud providers, universities, and content delivery networks each typically operate one or more autonomous systems. A company like Google manages multiple ASNs across different regions and services.

Interior vs. Exterior Routing

Within an AS, routers use interior gateway protocols such as OSPFOpen Shortest Path First. An interior gateway routing protocol that uses link-state advertisements and Dijkstra's algorithm to compute the shortest path within an autonomous system. or IS-IS to exchange routes. Between autonomous systems, BGPBorder Gateway Protocol. The routing protocol that exchanges reachability information between autonomous systems, effectively determining how data traverses the internet. Often called the "postal service of the internet." is the universal protocol. This division lets each AS optimize its internal routing independently while still participating in the global internet routing system.

Why AS Boundaries Matter

AS boundaries are where internet policy lives. Network operators use BGP route policies at AS edges to control which prefixes they advertise, to whom, and at what preference. Route leaks — where an AS accidentally advertises routes it should not — can cause significant traffic disruption, sometimes affecting entire regions of the internet.

Use WHOIS Lookup to identify which autonomous system owns a given IP address. The AS path shown in BGP routing tables reveals exactly which networks a packet traverses to reach its destination.