ASn

An Autonomous System Number (ASN) is a unique identifier assigned to an autonomous system (AS), a collection of IP networks under a single administrative authority that presents a common routing policy to the Internet. ASes exchange reachability information with other ASes using the Border Gateway Protocol (BGP), and the ASN is carried in BGP messages to identify the source and to influence routing decisions along the AS path.

AS numbers were originally 16-bit integers, but 32-bit numbers are now supported to accommodate growth. In practice, AS numbers can be represented in decimal or in AS dot notation, a form used by some routing software to display 32-bit values.

ASNs are allocated by regional Internet registries (RIRs) to organizations such as Internet service providers, data centers, universities, and large enterprises. A new ASN is typically requested when an organization needs to establish multihoming or participate in interdomain routing with multiple providers.

Private-use ASN ranges exist for internal routing within an organization; such numbers are not intended to be globally unique and should not be used on the public Internet.

An ASN is distinct from an IP address; possessing an ASN does not confer ownership of an IP address block, though the two resources are often managed together to enable interdomain routing. The ASN system is administered by the regional registries and underpins the policy and operation of BGP in global Internet routing.