See also: ACE

On Internet, a Autonomous Système ( Autonomous System or ACE ) is a whole of networks IP under the control of only one and even entity, typically a Fournisseur of access to Internet or a larger organization which have redundant connections with the remainder of the network Internet.

The autonomous concept of system is thus administrative and not technique. What characterizes the autonomous system, it is the unit of decision in its center. Thus, the internal policy of routing to the autonomous system (roads to be chosen in priority, filtering of the advertisements) is it coherent. The concept of system autonomous is thus opposed to that public network , as the Internet where different independent entities can make contradictory decisions.

Within an autonomous system, the protocol of routing is described as intern (for example, OSPF). Between two autonomous systems, the routing is external (for example BGP).

Each ACE is identified by an entirety of 16 bits (past recently with 32 bits, with the RFC 4893) which is used by the protocol of Routage Border Gateway Protocol which forms in particular the heart of Internet network. On Internet, this identifier is assigned by the organizations which allocate IP addresses, RIR. The numbers from 1 to 64511 are reserved for a public use (Internet) while those from 64512 to 65535 are reserved for a private use within an organization.

This entirety does not appear in the internal protocols of routing since, by definition, they are limited to only one ACE.