What is Autonomous System?
Also known as: AS, ASN
An Autonomous System (AS) is a group of IP networks under a single administrative routing policy, identified by a unique ASN (Autonomous System Number) for exterior gateway routing.
An Autonomous System is a collection of IP prefixes and routers that are managed by a single organization and present a common, coherent routing policy to the internet. The concept is fundamental to the operation of the Border Gateway Protocol (BGP). Without the AS abstraction, the global routing table would be impossible to manage; BGP uses the AS number as a key identifier to build a loop-free path across administrative boundaries.
Each Autonomous System is assigned a globally unique Autonomous System Number (ASN). Historically, ASNs were 16-bit numbers (range 1 to 64511 for public use), defined in RFC 1771. Due to exhaustion, RFC 6793 extended the format to 32-bit ASNs, written as either a plain integer or in the notation x.y, for example 23456 or 1.234. Private ASNs (64512 to 65535 for 16-bit, and 4200000000 to 4294967294 for 32-bit) are used for internal BGP peering and must not be advertised to the global internet.
The AS is the unit of routing policy enforcement. When ISPs, cloud providers, or large enterprises peer via BGP, they advertise their ASN along with the prefixes they originate. BGP path selection uses the AS path attribute (a sequence of ASNs a route has traversed) to detect loops and to prefer shorter paths. The ASN is also used in routing registries like RADB or RIPE IRR to filter legitimate announcements, mitigating route hijacks.
Key facts
- Defined in RFC 1930, updated by RFC 6793 to support 32-bit AS numbers.
- ASN 0 is reserved; ASN 23456 represents 32-bit ASNs in legacy BGP sessions.
- Public ASNs are assigned by regional internet registries (ARIN, RIPE, APNIC, etc.).
- BGP prepending artificially extends the AS path length to influence outbound traffic.
- A multi-homed AS must use BGP to announce its prefixes through multiple providers.
How it works in practice
Related terms
References
More in Networking & Routing
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A BGP path attribute that lists the sequence of autonomous system numbers a route has passed through, used for loop detection and path selection.
ASN
A globally unique 16 or 32 bit number assigned to an autonomous system for use in BGP routing between organizations on the Internet.
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