|  | 12.2 | Interior and Exterior Routing Protocols
| 12.2.1 | Autonomous system
An autonomous system consists of routers, run by one or more operators, that present a consistent view of routing to the external world. The Network Information Center (NIC) assigns a unique autonomous system to enterprises. This autonomous system is a 16 bit number. A routing protocol such as Cisco's IGRP requires that you specify this unique, assigned autonomous system number in your configuration.