What is ASN?
Also known as: Autonomous System Number
A globally unique 16 or 32 bit number assigned to an autonomous system for use in BGP routing between organizations on the Internet.
An Autonomous System Number (ASN) is a globally unique identifier assigned to an autonomous system (AS), which is a collection of IP prefixes and routing policies under the control of a single administrative entity. ASNs are used in the Border Gateway Protocol (BGP) to enable route exchange between different administrative domains on the Internet. Without ASNs, BGP would have no way to distinguish one network from another when exchanging reachability information.
Originally defined as a 16 bit field in RFC 1771 (1995), the ASN space allowed for 65,536 values. As the Internet grew, this space became insufficient, leading to the introduction of 32 bit ASNs in RFC 4893 (2007). The 32 bit format expanded the pool to over 4 billion identifiers. Today, both 16 bit and 32 bit ASNs coexist, with the reserved 16 bit ASN range 64512-65535 and the 32 bit range 4200000000-4294967294 designated for private use, analogous to private IP addresses.
ASN assignment is managed by the five Regional Internet Registries (RIRs: ARIN, RIPE NCC, APNIC, LACNIC, AFRINIC). An organization must justify its need for a public ASN, typically by demonstrating multi-homed connectivity to two or more distinct upstream providers. Large content providers, ISPs, and enterprises each use ASNs to announce their IP prefixes and enforce routing policies. The ASN is a fundamental component of inter-domain routing, sitting between the physical network layer and the higher-layer transport protocols.
Key facts
- ASNs are 16 or 32 bit numbers, with 16 bit values ranging from 1 to 64511, and 32 bit values from 131072 to 4294967295.
- Private ASNs (64512-65535 for 16 bit, 4200000000-4294967294 for 32 bit) are used for internal BGP only and must not appear on the public Internet.
- RFC 4893 (2007) standardized 32 bit ASNs, using a placeholder 16 bit ASN (23456) for backward compatibility.
- ASN 0 is reserved and should not be used in BGP operations.
- Each ASN is assigned by one of the five RIRs and globally unique for public use.
How it works in practice
Related terms
References
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