A network bridge, also known as a Layer 2 switch, is a hardware device used to create a connection between two separate computer networks or to divide one network into two. Both networks generally use the same protocol; Ethernet is an example of a protocol. Network devices include, but are not limited to, personal computers (PCs), printers, routers, switches, and hubs. Devices connected to a network through an Ethernet adapter card have what is known as a media access control (MAC) address, also called a physical or hardware address. It is this address that uniquely identifies a device on a bridge which can then determine which network the device is connected to.
Multiple network systems can be connected using a network bridge.
The main function of a network bridge is to forward data based on the MAC address of the sending and receiving devices. This operation helps eliminate what are known as collision domains. One way to define a collision domain is a network in which one device, also called a node, forces all other devices to listen when it transmits data packets. Another definition states that a collision domain exists when two or more devices attempt to transmit information at the same time. Networks running Carrier Sense Multiple Access/Collision Detection (CSMA/CD) should, in theory, be protected against collisions, but CSMA/CD can fail.
A network hub.
Whenever collisions occur, the efficient transmission of data packets is severely compromised. The more devices on a network trying to transmit data, the greater the chance of a collision. A network bridge can be used to segment a network into two, thus reducing the number of devices that compete for transmission privileges. For example, if network A has 20 devices, there is a chance that two or more of them will try to transmit data at the same time and cause a collision. By adding a bridge, you can split network A into networks A and B of 10 devices each.
Once the network bridge is integrated, it will begin to “listen” for data transmission made by devices on both networks. It does this by recording the MAC address of devices in a table that it generates automatically without being programmed to do so. When the first device transmits data, the bridge will add its MAC address to what is known as the forwarding table for future reference. The bridge also looks at the MAC address of the receiving or destination device. If it doesn’t appear in your table, the bridge will broadcast the data packet to all devices on both networks to locate the intended destination.
Routing tables are not created instantly; instead, the network bridge must wait until it receives a transmission from a device before learning its MAC address. The MAC addresses of the receiving devices must also be learned through broadcast, a lookup for the location of the destination. Once the destination responds, its address will also be added to the network bridge’s forwarding table. Eventually, all MAC addresses will be captured and data packets will be efficiently routed directly to their destinations. This will happen without all devices needing to listen to a streaming device.
When the sending and receiving devices are on the same network, forwarding of data packets does not occur. If they are on opposite networks, the bridge will forward the information. Avoiding collisions is not the only advantage of using bridges; they are also used to control the flow of information to maintain privacy. When a device transmits, the bridge sees it as a MAC address that belongs to one of two separate networks, and if the sending and receiving devices are on the same network, the data will not be forwarded. This is one way a network administrator can keep information private.
A network bridge can be used to connect a single copier or printer to two separate networks.
