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`my envelope in a neighborhood post office, they know to send it to the central post
`office. At the central post office, the clerks know from experience and current local
`conditions how to route it—whether it should go first
`to the airport, or if local
`weather is bad. whether it would be best sent by train to an airport in another city, or
`whatever. The same is true with TCP/{P packets. When a router, a special kind of
`computer that is the equivalent of a postal clerk, gets my TCP/IP packet, it either
`knows what route it should take or, if not, if it‘s like the clerk in the neighborhood
`rather than the central post office, it passes it on to another router that is able figure
`out the best route.
`
` g ?§.>'_
`
`
`TCP/IP: The TCP/IP stack (Transmission Control Protocol/Internet
`Protocol) is usually considered to be synonymous with the Internet. In
`fact, while all Internet links use IP at Layer 3, many transmissions use
`TCP’s sister protocol, UDP, in Layer 4.
`
`Packets to Frames
`
`Once the TCP/IP packets are created, they are in turn put into new packets that work
`with the computer’s local area network software and hardware. In this example, We’ll
`describe a network type called Ethernet. This system, which works only in a small
`area (typically up to fifty or so computers on one floor of a building), is like a com-
`pany’s internal mail service. The guys who deliver things around the office don’t deal
`with the outside world. Local mail goes in a company envelope and so does U.S.
`Mail. Ethernet doesn’t understand anything as complex as TCP/IP, and doesn’t need
`to. It just looks at the address on the kindpf packet it works with (actually, Ethernet
`calls its version of packets frames), takes it there, and drops it off; Just as the com-
`pany could use U.S. mail envelopes for its internal mail, it would be possible to have
`the LAN use just TCP/IP. We’ll see that there are a variety of reasons why it doesn’t,
`including the fact that the LAN’s simpler approach is cheaper and that TCP/IP and
`the various LAN protocols were all developed at different times by different compa-
`nies and organizations.
`
`
`LAN: A local area network (LAN) usually connects up to around fifty or
`so computers within a limited physical space, such as a small building
`or a floor of a larger one. The computers on a LAN are connected
`through a hub or a switch. LAN protocols, almost always Ethernet or
`Token Ring, operate at Layer 2 of the protocol stack.