A bridge is a device that controls network traffic flow on computer networks. A bridgereads the outermost section of data on the data packet, to tell where a message is going. It reduces the traffic on other network segments, since it does not send all packets. Bridges can be programmed to reject packets from particular networks. Bridging occurs at the data link layer of the OSI model, which means the bridge cannot read IP addresses, but only the outermost hardware address of the packet. In the case of ethernet networks, the bridge can read the ethernet data which gives the hardware address of the destination address rather than the IP address. Bridges forward all broadcast messages.

Only a special bridge called a translation bridge will allow two networks of different architectures to be connected. Bridges do not normally allow connection of networks with different architectures. The hardware address is also called the MAC (media access control) address. To determine the network segment a MAC address belongs to a bridge will use one of:

  • Transparent Bridging - They build a table of addresses (bridging table) as they receive packets. If the address is not in the bridging table, the packet is forwarded to all segments other than the one it came from. This type of bridge is used on ethernet networks.
  • Source route bridging - The source computer provides path information inside the packet. This is used on Token Ring networks.