A bridge is a device that connects and controls the flow of data between two LANs (local area networks) or two segments of the same LAN.
Bridges provide three main functions: (1) creating a bridging table to keep track of devices on each segment, (2) filtering packets based on their MAC addresses, i.e., forwarding packets whose destination MAC address is on a different segment of the network from their source and removing packets that do not need to be forwarded to other segments, and (3) dividing a single network into multiple collision domains, thereby reducing the number of collisions on each segment and effectively increasing its bandwidth.
Repeaters, whose main function is signal amplification, also connect two different network segments and pass data between them. Bridges incorporate the functionality of repeaters, but they additionally look at the packets and determine whether they should be allowed to pass through or not, whereas repeaters allow all data to pass through. Repeaters operate on the first layer of the OSI model, which provides the means for transmitting raw bits, but it is not concerned with MAC addresses, IP addresses and packets. Bridges operate at the data link layer, the second layer of the OSI seven layer model. Bridges also superficially resemble repeaters in appearance, i.e., it is a small box with two network connectors, but differ in that they also have indicator lights on them.
Hubs also connect network segments, but they are just essentially repeaters that can connect more than two segments, with models available ranging from four to several hundred connectors.
Bridges use the spanning tree protocol (STP) to decide whether to forward a packet through the bridge and on to a different network segment. STP serves two functions: to determine a main bridge, called a root, which will make all the bridging decisions and deal with all bridging problems, and to prevent bridging loops.
Created October 1, 2005.