A network bridge, also known as an Ethernet bridge, connects two segments of a network together. The segments are not independent entities, but are owned and managed by the same organisation. The purpose of the bridge is to divide a network into manageable sections.
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A network connects a number of computers together with one cable. The data passed over the cable is an electronic pulse that travels the length of the wire. More than one message pulsing on the cable would merge and interfere with each other. Only one device can transmit at any one time. If a network connects a large number of computers, each would have to wait a long time to get a turn applying data to the line. The main reason for segmenting networks is to reduce the number of devices connected to the same wire and thus reduce the wait time for transmission. A network bridge unites these segments.
Bridges operate with MAC addresses. On being introduced to a network, the bridge learns the MAC addresses of the devices on each segment to which it connects. It then keeps a table of these locations. The MAC address is the physical address of a computer and is hard-coded onto the computer's network adaptor.
Once the bridge has compiled its filtering table it operates by listening to the data packets travelling on each segment. If it receives a data packet where the source and destination addresses are on the same segment, it ignores it. This is called ''filtering." If the source and destination of the packet are on different segments, it passes the packet through to the segment containing the destination computer.
Network administrators often place two bridges between segments to guard against hardware failure. This causes a problem during the learning phase as each network bridge recognises the other as a path to an address. The two end up passing the data back and forth. This is called a bridging loop. The Spanning Tree Protocol resolves this problem by marking one as the primary bridge and the other as a reserve.
The bridge can also connect wireless and wired networks together. A wireless Ethernet bridge is a device for enabling non-wireless equipment to connect to Wi-Fi networks. In fact this is an external wireless network adaptor. However, it is marketed under the term "bridge."
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