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Bridge
A device that connects two local-area networks (LANs), or two segments of the same LAN.
At the time of their introduction in the 1980s, bridges connected and enabled packet forwarding between homogeneous networks. More recently, bridging between different networks has also been defined and standardized.
Bridging occurs at the link layer and is most commonly used to separate high-traffic areas on a LAN. Bridging relies heavily on broadcasting. Since a packet may contain no information other than the destination address, and that implies nothing about the path that should be used, the only option may be to send the packet everywhere (broadcasting)! This is one of bridging's most severe limitations, since this is a very inefficient method of data delivery, and can trigger broadcast storms.
In a bridged network, no correspondence is required between addresses and paths. Put another way, addresses don't imply anything about where hosts are physically attached to the network. Any address can appear at any location. In contrast, routing requires more thoughtful address assignment, corresponding to physical placement.
With bridging, then, data is forwarded without the use of protocol addresses. An Ethernet hub is a kind of bridging device. When a packet arrives on one interface, the hub will copy it onto all the remaining interfaces. Switches do the same thing more intelligently by remembering information that allow them to find the correct output cable. In general, bridging is used by devices that do not understand the routing protocols involved.
Related terms:
LAN, Network, Router
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