8.1 Autonomous Systems
8.1.1 Overview of autonomous systems
An internetwork is a group of smaller, independent networks. Each of these smaller networks may be owned and operated by a different organization: a company, a university, a government agency, or some other group. The Internet is one example of a single, albeit immense, internetwork.

Not surprisingly, the operators of these individual networks desire autonomy, or self-administration, over their own systems. Because the routing and security policies of one organization may conflict with the policies of another, internetworks are divided into domains, or autonomous systems. Each AS typically represents an independent organization and applies its own unique routing and security policies. EGPs facilitate the sharing of routing information between autonomous systems. (See the Figure).

An AS is any set of routers that share similar routing policies and operate within a single administrative domain. An AS can be a collection of routers running a single IGP, or it can be a collection of routers running different protocols all belonging to one organization. In either case, the outside world views the entire AS as a single entity.

Each AS has an identifying number, assigned by an Internet registry or a service provider, between 1 and 65,535. AS numbers within the range, 64,512 through 65,535 are reserved for private use (similar to RFC 1918 IP addresses). Because of the finite number of available AS numbers, an organization must present justification of its need before it will be assigned an AS number.

Today, the Internet Assigned Numbers Authority (IANA) is enforcing a policy whereby organizations that connect to a single provider and share the provider's routing policies use an AS number from the private pool (64,512 to 65,535). These private AS numbers appear only within the provider's network and are replaced by the provider's registered number upon exiting the network. Thus, to the outside world, several individual networks are advertised as part of one service provider's network. In principal, this process is similar to NAT (see Chapter 2, IP Addressing).

During the early days of the Internet, an EGP called EGP version 3 (EGP3 - not to be confused with EGPs in general) was used to interconnect autonomous systems. Currently, BGP4 is the accepted standard for Internet routing and has essentially replaced the more limited EGP3.

The following sections detail the different types of autonomous systems: single-homed, multihomed nontransit, and multihomed transit. In addition to defining these three types of systems, these sections will examine BGP's role in connecting each type of AS to an ISP.