The Internet is not operated as one routing domain. Networks under a common administration and routing policy are grouped into autonomous systems (ASes).

Inside an AS, operators can optimize routes according to internal metrics. Between ASes, routing must also respect business and policy relationships: a technically short path may not be an acceptable path.

An autonomous system number (ASN) identifies an AS in interdomain routing. Networks advertise which IP prefixes they can reach, and neighboring ASes decide which advertisements to accept and propagate.

This creates a hierarchy of routing problems: an interior routing protocol computes paths within one administrative domain, while interdomain routing exchanges reachability and policy between domains.

Understanding ASes explains why Internet routing is not simply one global shortest-path calculation.