Fortunately, this is a Nutshell book, so I don't have to do anything more than give a brief introduction to the Border Gateway Protocol (BGP), one of the most complex topics in network routing. Covering BGP in any detail could easily require hundreds of pages. Unlike the other routing protocols we've discussed, BGP is an exterior routing protocol, which means it routes traffic between different autonomous systems. Its primary use is on the Internet backbone; it shouldn't be used on most networks that are connected to the Internet. However, if you have more than one Internet service provider or your network is multihomed, you must use BGP.
BGP is a successor to EGP, which had many limitations. BGP's main new feature was Classless Interdomain Routing (CIDR), which rescued the almost-exhausted Internet IP address space. The current version of BGP is BGP4; it's unlikely that you'll need to know about earlier versions.