A network bridge connects two computer networks or divides a network, forwarding data based on MAC addresses to reduce collisions. It generates a table of MAC addresses and can maintain privacy by not forwarding data between devices on the same network.
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 a network into two. Both networks usually 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 via an Ethernet adapter card have a so-called MAC (Media Access Control) address, also called a physical or hardware address. It is this address that uniquely identifies a device to a bridge which can then determine which network the device is connected to.
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 to eliminate the so-called collision domains. One way to define a collision domain is a network in which a device, also called a node, forces every other device to listen when it transmits data packets. Another definition states that a collision domain exists when two or more devices try to transmit information at the same time. Networks running Carrier Sense Multiple Access/Collision Detection (CSMA/CD) should, in theory, be protected from collisions occurring, but CSMA/CD can fail.
Whenever collisions occur, the efficient transmission of data packets is greatly affected. The more devices that are on a network attempting to transmit data, the greater the chance of a collision occurring. A network bridge can be used to segment a network into two, thereby reducing the number of devices competing for broadcast privileges. For example, if network A has 20 devices, it is likely that two or more of them are trying to transmit data at the same time and cause a collision. If a bridge is added, it can divide network A into networks A and B of 10 devices each.
Once the network bridge is incorporated, it will begin to “listen” to data transmissions made by devices on the two networks. It does this by registering the devices MAC address in a table that it automatically generates 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 examines the MAC address of the destination or receiving device. If it doesn’t appear in its table, the bridge will broadcast the data packet to all devices on both networks to locate its intended destination.
Forwarding tables are not built instantly, rather the network bridge has to wait until it receives a broadcast from a device before it can learn its MAC address. The MAC addresses of the receiving devices must also be learned via broadcast, a search for the location of the destination. Once the destination replies, its address is also added to the forwarding table of the network bridge. Eventually, all MAC addresses will be captured and data packets will be efficiently routed directly to their destination. This will happen without all devices having to listen to a transmitting device.
When the sending and receiving devices are on the same network, no forwarding of data packets takes place. If they are on opposite networks, the bridge will forward the information. Collision avoidance isn’t the only benefit of using bridges; they are also used to control the flow of information to maintain privacy. When a device transmits, it is seen by the bridge 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 confidential.
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