tance is a few hundred meters. With help of the switching devices (switches or bridges) or routers, LANs can be interconnected to make up a wide-area corporate network. The bridges or switches interconnect separate LAN segments and switch frames from one segment to another with the help of a local hardware address that is stored in the interface unit of each computer. Routers are devices that use network layer addresses for the routing of packets and they are used to con- nect LANs to other networks, for example, to the Internet. Routers can also be used to interconnect LANs that use different technologies. The basic structures of the two most common LANs, Ethernet and token ring, are presented in Figure 6.25. The original principle of all LAN networks is that all computers are connected to the same physical cable and they use it in turn. Information is sent in long frames that include the hard- ware addresses of both the source and the destination. These addresses are unique to each interface card plugged into a computer. Each computer lis- tens to the cable and receives the frames that contain its own identification as a destination address. Special protocols are standardized to make sure that only one computer transmits at a time. The complex standards of LANs specify OSI layer 1, the physical layer, and the so-called medium access sublayer (MAC) of layer 2 (the data link layer). The basic task of these protocols is to connect a Ethernet Token Ring ISO 8802-3/IEEE 802.3 ISO 8802-5/IEEE 802.5 The structure of a data frame: Addresses Error check frames at 16 Mbps 10 Mbps Message data Termination Figure 6.25 LAN structure. Data Communications computer to another via a shared medium as if they were connected by a point-to-point cable. The most common LAN is the Ethernet, which has been standardized as ISO 8802-3 or ANSI/IEEE 802-3. Its original principle was invented by Metcalfe and Broggs and developed by Digital, Intel, and Xerox. It was called (DIX) Ethernet, and it became the de facto standard for LANs. The st