Autonomous System

An autonomous system (AS) is a collection of networks under the administrative control of a single entity that presents a common routing policy to the Internet. In the figure, companies A, B, C, and D are all under the administrative control of ISP1. ISP1 "presents a common routing policy" for all of these companies when advertising routes to ISP2.

The guidelines for the creation, selection, and registration of an autonomous system are described in RFC 1930. AS numbers are assigned by the Internet Assigned Numbers Authority (IANA), the same authority that assigns IP address space. You learned about IANA and its Regional Internet Registries (RIRs) in a previous course. The local RIR is responsible for assigning an AS number to an entity from its block of assigned AS numbers. Prior to 2007, AS numbers were 16-bit numbers, ranging from 0 to 65535. Now 32-bit AS numbers are assigned, increasing the number of available AS numbers to over 4 billion.

Who needs an autonomous system number? Usually ISPs (Internet Service Providers), Internet backbone providers, and large institutions connecting to other entities that also have an AS number. These ISPs and large institutions use the exterior gateway routing protocol Border Gateway Protocol, or BGP, to propagate routing information. BGP is the only routing protocol that uses an actual autonomous system number in its configuration.

The vast majority of companies and institutions with IP networks do not need an AS number because they come under the control of a larger entity such as an ISP. These companies use interior gateway protocols such as RIP, EIGRP, OSPF, and IS-IS to route packets within their own networks. They are one of many independent and separate networks within the autonomous system of the ISP. The ISP is responsible for the routing of packets within its autonomous system and between other autonomous systems

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