r all practical purposes, a gateway is a router. Router is the term I will generally use in this book, although you will also see the phrase default gateway. In the olden days of data communication, a "gateway" was a device that translated one protocol to another. For example, if you had a device that converted a serial link into a parallel link, that device would be called a gateway. Similarly, a device that converted Ether- net to Token Ring might be called a gateway. Nowadays, such devices are called media converters. (This wouldn't be the first time I've been accused of harkening back to the good old days. Pull up a rocker here on the porch, and have a mint julep while I spin you a yarn.) Routers usually communicate with each other by means of one or more routing pro- tocols. These protocols let the routers learn information about networks other than the ones directly connected to them. Network devices used to be limited to bridges and routers. Bridges, hubs, and switches operated only on layer two of the OSI stack, and routers only on layer three. Now these devices are often merged into single devices, and routers and switches often operate on all seven layers of the OSI stack. In today's world, where every device seems to be capable of anything, when should you pick a router rather than a switch? Routers tend to be WAN-centric, while switches tend to be LAN-centric. If you're connecting T1s, you probably want a router. If you're connecting Ethernet, you probably want a switch. Routing Tables Routing is a fundamental process common to almost every network in use today. Still, many engineers don't understand how routing works. While the Cisco certifica- tion process should help you understand how to configure routing, in this section, I'll show you what you need to know about routing in the real world. I'll focus on the foundations, because that's what most engineers seem to be lacking-we spend a lot of time studying the latest technologies, and sometimes forget the core principles on which everything else is