Bridges are network devices used to extend the network by extending the broadcast domain. Bridges can connect two networks that use the same protocol to communicate (Ethernet to Ethernet, FDDI to FDDI, Token Ring to Token Ring). Bridges replicate frames received on one port on one side of the bridge and broadcast the frame out the other side of the bridge.
Intelligent Bridges can learn the MAC addresses on the networks on either side of the bridge. If a frame on one side of the bridged network needs to reach a host on the other side of the bridge, the bridge will forward the frame. If a frame is transmitted and the bridge cannot determine where the destination host resides, it will broadcast the packet on the opposite side.
Switches are a type of intelligent multi-port bridge.