|  | 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.