be forwarded to a gateway. The gateway is responsible for determining how to get the packet to where it needs to be. RFC 791, titled INTERNET PROTOCOL, defines gateway thusly: 2.4. Gateways Gateways implement Internet protocol to forward datagrams between networks. Gate- ways also implement the Gateway to Gateway Protocol (GGP) [7] to coordinate routing and other Internet control information. In a gateway the higher level protocols need not be implemented and the GGP functions are added to the IP module. When a station on a network sends a packet to a gateway, the station doesn't care how the packet gets to its destination-just that it does (at least, in the case of TCP). Much like a letter in the postal system, each packet contains its source and destination addresses SO routing can be accomplished with ease. In the realm of semantics and IP, a gateway is a device that forwards packets to a destination other than the local network. For 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 Ethernet 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 hark- ening 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 proto- cols. 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 2 of the OSI stack, and routers only on Layer 3. Now these devices are often merged in