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A bridge is a LAN connection device with two or more ports that forwards frames from one LAN (local area network) segment to another. In the past, the bridge was a small box with several LAN connectors or a server with several network interface cards. Today, bridges are more likely to appear in the form of switching devices, which are technically multiport bridges. Each port provides a separate LAN connection that is bridged to the other ports.

The bridge provides several important functions:

• It allows a single LAN to be extended to greater distances.


• You can join different types of network links with a bridge while retaining the same broadcast domain. For example, you can bridge two distant LANs with bridges joined by fiber-optic cable.


• A bridge forwards frames, but a filtering mechanism can be used to prevent unnecessary frames from propagating across the network.


• They provide a barrier that keeps electrical or other problems on one segment from propagating to the other segment.


• A bridge isolates each LAN from the collisions that occur on other LANs. Thus, it creates separate collision domains within the same broadcast domain.

The last point is important. On Ethernet networks, collisions occur when two nodes attempt to transmit at the same time. As more nodes are added to a network, collisions increase. A bridge can be used to divide a network into separate collision domains while retaining the broadcast domain. A broadcast domain is basically a LAN as compared to an internetwork, which is multiple LANs connected by routers. In a broadcast domain, any node can send a message to any other node using data link layer addressing, while a routed network requires internetwork addressing.