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Network Layer Devices

Three devices operate at the Network layer:

■    Routers

■    Brouters

■    Layer 3 Switches

The Router

The router is the device that connects multiple networks or segments to form a larger internetwork. It is also the device that facilitates communication within this internetwork. It makes the choices about how best to send packets within the network so that they arrive at their destination.

Several companies manufacture routers, but probably the two biggest names in the business are Bay Networks and Cisco. Bay Networks is a conglomeration of smaller networking companies bought out by networking giant Synoptics. Cisco has always been a built-from-the-ground-up router company. Both companies make other products, to be sure, but their bread and butter is routing technologies.

Routers have many functions other than simply routing packets. Routers can connect many small segments into a network, as well as connect networks to a much larger network, such as a corporate WAN or the Internet. Routers can also connect dissimilar lower-layer topologies. For example, you can connect an Ethernet and a Token Ring network using a router. Additionally, with added software, routers can perform firewall functions and packet filtering.

Routers are probably the most complex devices on a network today. Consequently, they are likely to be the most expensive. But simple low-end routers have been introduced by Bay Networks, Cisco, and other companies

in the sub-$1,000 range that make Internet connectivity more affordable. Hub vendors have begun to introduce basic intranetwork routing functionality into their products as well. You will learn more about that later in this chapter when we discuss switches.

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