8.3 The Data Link Layer
8.3.1 Bridges, what they are and why they are used
Bridges

Bridges are devices which connect two different networks, or network segments together and filter traffic from each network. The bridge builds a table of physical (hardware) addresses, learning the hosts, which exists on each of its ports. The bridge examines the destination MAC address of each frame; if the destination address is local (on the same bridge port, based on the bridging table), the frame is not sent. However, if the destination MAC address is of a different bridge port from the source address, the frame is forwarded to the non-local destinations. Bridges provide connectivity with Layer 2 filtering. Some bridges will connect networks of differing LAN technologies (like Ethernet to Token Ring). Since the bridge operates at layer 2, it forwards all upper level protocols.