What is ASN?

An ASN (Autonomous System Number) is the unique number that identifies an autonomous system, a network with its own routing policy, in BGP.

Every BGP router is configured with its own ASN and the ASN it expects each neighbour to have; if the number in a neighbour's OPEN message doesn't match, the session doesn't come up. ASNs were originally 16-bit (0–65535). That pool ran short, so RFC 6793 extended them to 32 bits, and modern routers negotiate 4-byte ASN support when the session opens. A few ranges are worth knowing:

• 64496–64511 and 65536–65551 are reserved for documentation and examples.
• 64512–65534 and 4200000000–4294967294 are private, like RFC 1918 addresses: fine inside your own network, never on the public internet.
• 0, 65535 and 4294967295 are reserved.

Public ASNs are assigned by a Regional Internet Registry, usually to an organisation that connects to more than one provider and needs its own routing policy. A customer with a single upstream can often use a private ASN, which the provider removes before passing routes on. Each time a route crosses an AS boundary, the sender's ASN is added to the route's AS_PATH, which is how BGP detects loops and measures distance. A common confusion is between the AS (the network) and the ASN (its number); in conversation the two are used almost interchangeably, but see AS for the network itself. Large 32-bit ASNs are normally written as a plain number (asplain), although some older documentation uses a dotted "asdot" form. The BGP Fundamentals lesson walks through the ranges and how ASNs appear in sessions and paths.