Autonomous system numbers (ASNs) is a unique identifier that is globally available and allows its autonomous system to exchange routing information with other systems. An Internet Assigned Numbers Authority (IANA) globally assigned autonomous system is a group of networks under the administrative control of a single body that presents a common routing policy to the Internet. The guidelines for the creation, selection, and registration of an autonomous system are described in RFC 1930.
Global autonomous system numbers are assigned by IANA, the similar authority that assigns IP address space. The local regional Internet registry (RIR) can assign an autonomous system number to an entity from its block of assigned autonomous system numbers.
Usually, Internet Service Providers (ISPs) use autonomous System Numbers to control routing within their networks and to exchange routing information with other Internet Service Providers (ISPs). They use the exterior gateway routing protocol, Border Gateway Protocol (BGP), to propagate routing information. BGP is the only routing protocol that uses an actual autonomous system number in its configuration.
Autonomous System Numbers Format
The Autonomous System has two different formats to represent: 2-byte and 4-byte format. A 2-byte format provides 65,536 Autonomous System number ranges from 0 to 65535. The Internet Assigned Numbers Authority (IANA) reserved 1,023 numbers (64512 to 65534) for private use.
A 4-byte Autonomous System format provides for 4,294,967,296 ASNs ranges from 0 to 4294967295. The Internet Assigned Numbers Authority (IANA) reserved a block of 94,967,295 ASNs ranges from 4200000000 to 4294967294 for private use.
Autonomous System Numbers in EIGRP
EIGRP uses the “router eigrp <autonomous-system>” command in global configuration mode to enable the EIGRP process. The autonomous system number uses in the EIGRP configuration are not related to the Internet Assigned Numbers Authority (IANA).
The autonomous system number uses for EIGRP configuration are only important to the EIGRP routing domain. It helps routers keep track of multiple, running instances of EIGRP. The autonomous system number is required because it is possible to have more than one instance of EIGRP running on a network. Each instance of EIGRP can be configured to support and exchange routing updates for different networks.