The network structure its architecture (Figure 6.25) typifes what are known as wide area networks (WANs). The nodes, and users for that matter, are spread geographically over long distances. "Long" has no precise defnition, and is intended to suggest that the communication links vary widely. The Internet is certainly the largest WAN, spanning the entire earth and beyond. Local area networks, LANs, employ a single communication link and special routing. Perhaps the best known LAN is Ethernet. LANs connect to other LANs and to wide area networks through special nodes known as gateways (Figure 6.27). In the Internet, a computer's address consists of a four byte sequence, which is known as its IP address (Internet Protocol address). An example address is 128.42.4.32: each byte is separated by a period. The first two bytes specify the computer's domain (here Rice University). Computers are also addressed by a more human-readable form: a sequence of alphabetic abbreviations representing institution, type of institution, and computer name. A given computer has both names (128.42.4.32 is the same as soma.rice.edu). Data transmission on the Internet requires the numerical form. So-called name servers translate between alphabetic and numerical forms, and the transmitting computer requests this translation before the message is sent to the network.
The gateway serves as an interface between local area networks and the Internet. The two shown here translate between LAN and WAN protocols; one of these also interfaces between two LANs, presumably because together the two LANs would be geographically too dispersed.
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