BGP and ASNs: how Internet networks exchange routes

An autonomous system is a group of IP networks and routers operated under a common routing policy. It can be assigned an ASN. An ASN is not a server or user identifier; it identifies a participant in interdomain routing.

What BGP and ASNs tell you

Registration information about an ASN can be compared with the autonomous-system overview and IP data. One operator can use several ASNs, while one online service can depend on several network operators.

How to inspect the route between networks

BGP distributes reachability for IP prefixes together with route attributes. AS_PATH records autonomous systems traversed by an advertisement, but it is neither a geographic map nor a guaranteed record of every packet's path.

Operator policy, peering and backup links affect route selection. A destination can therefore keep the same IP address while the route used to reach it changes. For an additional check, use 2ip WHOIS/RDAP lookup.

What BGP does not tell you

Compare the ASN and prefix with 2ip IP information, then inspect the observed path with traceroute. Do not diagnose an outage from AS_PATH length alone: BGP describes the control plane, while traceroute provides a limited data-plane observation.

The base BGP-4 specification is RFC 4271.