An autonomous system, or AS, is the fundamental administrative unit of internet routing. It is a network, or a connected group of networks, that presents a single, consistent routing policy to the rest of the internet. The defining document is RFC 1930, “Guidelines for creation, selection, and registration of an Autonomous System (AS),” written by J. Hawkinson of BBN Planet and T. Bates of MCI and published in March 1996. It defines an AS as “a connected group of one or more IP prefixes run by one or more network operators which has a SINGLE and CLEARLY DEFINED routing policy.”
The emphasis in that definition is on policy rather than ownership. RFC 1930 stresses that an autonomous system is about routing policy, not administrative grouping, and that the same AS may be operated by more than one network operator as long as they present one coherent policy to their neighbors. This complements the definition in BGP’s own specification, RFC 4271, which describes an AS as a set of routers “under a single technical administration” maintaining “a single coherent interior routing plan” while using an inter-AS protocol to reach other systems. The two documents agree: an AS is the boundary at which a network’s internal routing choices become external commitments to its neighbors.
Each autonomous system is identified by an Autonomous System Number, or ASN, a globally unique integer used in exterior routing exchanges. RFC 1930 notes that ASNs were originally defined as 16-bit integers, which limited the total to 65,535 distinct numbers, and it reserves the range 64512 through 65535 for private use within organizations that do not need a globally unique identity. As the internet grew past the limits of 16-bit numbering, the registries moved to 32-bit ASNs, but the role of the number was unchanged: it names the policy domain that originates and forwards routes.
Not every network needs its own AS. RFC 1930 explains that you primarily need one when you are multi-homed, connecting to more than one provider with routing policies that differ from theirs, or when you must exchange external routing information under a policy distinct from your neighbors. A single-homed site usually does not need a separate ASN, because its prefixes can simply live inside its provider’s autonomous system. The guideline exists to keep the global routing table from filling with unnecessary AS numbers.
Autonomous systems are the vertices in the internet’s routing graph, and BGP is the protocol that draws the edges. When BGP advertises a route, it carries the list of autonomous systems the route passes through, the AS path. Loop detection, policy enforcement, and the entire structure of inter-domain routing rest on this idea that the internet is not one network but a federation of autonomous systems, each with its own policy, exchanging reachability across well-defined boundaries.