uting is a term with multiple meanings in different disciplines. In general, it refers to determining a path for something. In telecom, a call may be routed based on the number being dialed or some other identifier. In either case, a path is determined for the call. Mail is also routed-I'm not talking about email here (though email is routed too)- but rather, snail mail. When you write an address and a zip code on a letter, you are providing the means for the post office to route your letter. You provide a destina- tion and, usually, a source address, and the post office determines the best path for the letter. If there is a problem with the delivery of your letter, the return address is used to route it back to you. The exact path the letter takes to get from its source to its destination doesn't really matter; all you care about is that it (hopefully) makes it in a timely fashion, and in one piece. In the IP world, packets or frames are forwarded on a local network by switches, hubs, or bridges. If the address of the destination is not on the local network, the packet must 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 a gateway thusly: 2.4. Gateways Gateways implement internet protocol to forward datagrams between networks. Gateways 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 that routing can be accomplished with ease. In the realm of semantics and IP, a gateway is a device that forwards packets to a des- tination other than the local network. Fo