8.1 Autonomous Systems
8.1.3 Characteristics of an autonomous system
An AS is any set of routers with a consistent routing policy running within a single administrative domain. An AS could be a collection of routers running a single Interior Gateway Protocols (IGP), or it could be a collection of routers running different protocols all belonging to one organization. In either case, to the outside world the whole AS is viewed as one single entity. Each AS has an identifying number, assigned by an Internet Registry or a service provider, between 1 and 65535. AS numbers within the range 64512 through 65535 are reserved for private use. Because of the finite number of available AS numbers, justification should be presented before an organization is given an AS number.

Today, the ARIN.NET enforces a policy whereby organizations connecting to a single provider and sharing the same routing policies as their providers use an AS number from the private pool. These private ASs are stripped at the provider level. Thus, to the outside world, several individual networks are advertised as part of one service provider's network. Organizations connecting to multiple providers can request an AS number from the public pool. Organizations that qualify for getting an AS number should provide an AS name, an organization name, and a technical point of contact. Routing information between ASs is exchanged via an exterior gateway protocol such as BGP4, as illustrated in the Figure.

By defining Autonomous Systems, the Internet gains the manageability and scalability necessary for sustained growth. Each AS can implement its own set of rules, policies, and IGPs independent of other ASs.