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Special-purpose devices are used to provide connections between networks and systems. Sometimes the terms gateway and router are used interchangeably. Strictly speaking, gateway describes a system that sends messages between different types of networks; a router sends messages between networks of the same type.
In this text, router is routinely used to describe any device that takes messages from one network and passes them through to another network. The router contains enough intelligence to know whether the message received must be forwarded to another network or a router.
Routers operate at the Network layer and are usually associated with a protocol, such as IP or IPX. Most routers that route IPX traffic can route IP traffic as well. The router is used to connect multiple local and wide area networks. It provides a method of sharing data between networks. Also, because a router works at the Network layer, it can help reduce broadcast traffic.
If one network uses a lot of different protocols and another network uses only IP, a router that routes only IP messages is needed if those two networks are to communicate. The router prevents messages from being placed on a network that can’t manage them.
Bridges, on the other hand, can be used to interconnect local and wide area networks; they share information regardless of protocol. A bridge allows two interconnected networks to have many different protocols on them at the same time. The messages forwarded by a bridge usually don’t contain any further routing information. The messages are usually left undisturbed.
One drawback of bridges is that all network broadcast and multicast messages from all interconnected networks are seen on all legs connected by a bridge. This results in a lot of overhead related to network update messages. Also, a bridge forwards messages only to network addresses on the other side of the bridge, but it can forward all network protocols and broadcast messages.
Routers and bridges are used to share information between networks. The appropriateness of each is determined by networking requirements, the protocols involved, network capacity, and user demands. The proper selection of components can help a network operate efficiently, allow for future growth, and help ensure continued reliability.
NOTE: Use bridges only if multiple protocol packets are to be shared. Otherwise, a router is a better choice because it helps reduce network overhead.
You can find more information about TCP/IP in the following chapters:
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