AS number
An autonomous system number identifies the network that announces an address prefix, and the AS path shows which networks the route crosses.
Definitions only — a term explained here is not a service we sell.
An autonomous system number (ASN) identifies a routing domain: one network, or a set of address prefixes administered under one routing policy. It names the network that announces a prefix; it is not a location or a company registration.
The AS number space is 32-bit; RFC 6793 specifies the four-octet space. Registry country is metadata, not a service map.
Why buyers care
The ASN tells you who carries the traffic and how many networks stand in the path. A short path is not automatically better: two routes to one prefix, three ASNs against five, resolve on the shorter path by default, regardless of congestion or capacity. An unexpected origin is a hijack until proven otherwise. The AS numbers guide covers the registry and peering practice.
How the AS path is built
- AS64500 (documentation)
- AS64501 (documentation)
- AS64502 (documentation)
- Internet
Text description
The first ASN is the neighbor that handed you the route; the last is the origin, the network that announced the prefix. A looking glass shows the path without asking the operator.