Bridges became commercially available in the early 1980s. At the time of their introduction their function was to connect separate homogeneous networks. Subsequently, bridging between different networks – for example, Ethernet and Token Ring – has also been defined and standardised. Bridges are data communications devices that operate principally at Layer 2 of the OSI reference model. As such, they are widely referred to as data link layer devices.
Bridges map the Ethernet addresses of the nodes residing on each network segment and allow only necessary traffic to pass through the bridge. When a packet is received by the bridge, the bridge determines the destination and source segments. If the segments are the same, the packet is dropped (filtered); if the segments are different, then the packet is forwarded to the correct segment. Additionally, bridges do not forward bad or misaligned packets.
Bridges are also called store-and-forward devices because they look at the whole Ethernet packet before making filtering or forwarding decisions. Filtering packets, and regenerating forwarded packets enables bridging technology to split a network into separate collision domains. This allows for greater distances and more repeaters to be used in the total network design.
Most bridges are self-learning task bridges; they determine the user Ethernet addresses on the segment by building a table as packets are passed through the network. This self-learning capability, however, dramatically raises the potential of network loops in networks that have many bridges. A loop presents conflicting information on which segment a specific address is located and forces the device to forward all traffic.
By the mid-1990s, switching technology had emerged as the evolutionary heir to bridging based internetworking solutions. Superior throughput performance, higher port density, lower per-port cost, and greater flexibility have contributed to the emergence of switches as a replacement technology for bridges and as a complementary technology to routing.