An Autonomous System Number (ASN) is a unique identifier allocated to each Autonomous System (AS), which is a collection of IP routing prefixes under the control of one or more network operators that presents a unified routing policy to the internet. ASNs are used in Border Gateway Protocol (BGP) routing to facilitate the exchange of routing information between different ASes, allowing for proper routing of data across the internet.
ASNs come in two formats: 2-byte and 4-byte. A 2-byte ASN is a 16-bit number, providing for 65,536 possible ASNs (0 to 65535), with a subset reserved for private use. A 4-byte ASN is a 32-bit number, significantly increasing the number of available ASNs to 4,294,967,296 (0 to 4294967295), with a block reserved for private use as well. The transition from 2-byte to 4-byte ASNs was proposed by the Internet Engineering Task Force (IETF) in 2007 to accommodate the growing need for more ASNs.
Different types of ASes that may be assigned an ASN:
Multi-homed AS: Connects to two or more ASes to maintain internet connectivity even if one connection fails.
Stub AS: Connects to only one other AS and may have private connections not visible to the rest of the internet.
Transit AS: Acts as a link between two or more ASes, allowing data to pass through it, including data from unassociated networks.
ASNs are assigned by the Internet Assigned Numbers Authority (IANA) to regional Internet registries (RIRs), which in turn allocate them to local Internet registries (LIRs), end-user organizations, and network operators. The assignment process involves demonstrating a unique routing policy or the need for a multi-homed site.