Routers are commonly used to interconnect those

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Unformatted text preview: ost commonly used approach is to make use of common high-level protocols for moving data between common layers on a communications model such as the OSI or the IP suites. Internetworking tools, such as bridges, routers, brouters, and gateways, make extensive use of this approach. As described below, each of these tools has strengths, weaknesses, and specific applications in internetworking. These tools are "blackbox" internetworking technologies that enable interconnection of similar or dissimilar networks to form a single network system. Bridges Bridges operate at the bottom two layers of the OSI model (physical and data link). Therefore, they are used to connect networks that use the same communication protocols above the data-link layer but may or may not use the same protocols at the physical and data-link layers. For example, bridges may be used to connect two networks, one of which uses fiber-optic communication medium and the other uses coaxial cable; or one of which uses Ethernet technology and the other uses Token Ring technology. But both networks must use the same high-level protocols (e.g., TCP/IP or XNS) to communicate. The similarity of higher-level protocols implies that bridges do not modify either the format or the contents of the frames when they transfer them from one network segment to another (they simply copy the frames). Hence bridges feature highlevel protocol transparency. They can transfer data between two network segments over a third segment in the middle that cannot understand the data passing through it. As far as the bridge is concerned, the intermediate segment exists for routing purposes only. Bridges are intelligent devices in the sense that they use a process of learning and filtering in data forwarding to keep network traffic within the segment of the network to which it belongs. Therefore, bridges are also useful in network partitioning. When the performance of a network segment degrades due to excessive network traffic, it can b...
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