Content

| 9.1 |  | Autonomous Systems |
| 9.1.1 |  | 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. These organizations can include a company, a university, a government agency, or some other group. The Internet is one example of a single, although immense, internetwork.

The operators of these individual networks desire independence 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 (AS). 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.

An AS is a group 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 Autonomous System as a single entity.

Each AS has an identifying number that is assigned by an Internet registry or a service provider. This number is between one (1) and 65,535. AS numbers within the range of 64,512 through 65,535 are reserved for private use. This is 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. Therefore, 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 Module 2, IP Addressing.

During the early days of the Internet, an Exterior Gateway Protocol, EGP version 3, was used to interconnect autonomous systems. EGP3 should not be confused with EGPs in general. 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 such as single homed, multihomed nontransit, and multihomed transit. In addition to defining these three types of systems, the following sections will examine the role BGP has in connecting each type of AS to an ISP.