There are times when it is necessary to break up a large LAN into smaller, more easily managed segments. This decreases the amount of traffic on a single LAN and can extend the geographical area past what a single LAN can support. The devices that are used to connect network segments together include bridges, switches, routers, and gateways. Switches and bridges operate at the Data Link layer of the OSI model. The function of the bridge is to make intelligent decisions about whether or not to pass signals on to the next segment of a network.

When a bridge receives a frame on the network, the destination MAC address is looked up in the bridge table to determine whether to filter, flood, or copy the frame onto another segment. This decision process occurs as follows:

• If the destination device is on the same segment as the frame, the bridge blocks the frame from going on to other segments. This process is known as filtering.
• If the destination device is on a different segment, the bridge forwards the frame to the appropriate segment.
• If the destination address is unknown to the bridge, the bridge forwards the frame to all segments except the one on which it was received. This process is known as flooding.

If placed strategically, a bridge can greatly improve network performance.     

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