BGP - Border Gateway Protocol

On the Internet, there are what are known as Interior Gateway Protocols (IGP) and Exterior Gateway Protocols (EGP). The difference is simplest to illustrate by example.

Take ISP X, who has uplink connections to Sprint and UUnet. ISP X has a complex network of routers and modem servers and also has T1 and ISDN connections to customers of theirs. They use an Interior routing protocol called OSPF to allow all of their networking equipment to communicate amongst themselves to describe the network in great detail. However, their uplinks, Sprint and UUnet, do not need or want to see that level of detail, so ISP X communicates with Sprint and UUnet using an Exterior routing protocol, BGP.

In the above example, ISP X is an Autonomous System (AS). In other words, the admins of ISP X have control over networking equipment belonging to them. There is a clear delineation between ISP X's network and UUnet's network. Hence, ISP X and UUnet are two different Autonomous Systems.

Today, BGP4 is the only standard protocol for communicating between autonomous systems on the Internet. BGP4 is the fourth and current revision of the protocol. Anyone wanting to use BGP on the Internet must have an Autonomous System Number (ASN) assigned to them by ARIN. They currently charge money for this service.