A bridge in networking functions primarily to connect two or more network segments, typically LANs, creating a single, larger network. It does this by analyzing the destination MAC address of data packets and forwarding them only to the segment where the destination device is located, improving network efficiency.
Key Functions of a Network Bridge:
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Connecting Network Segments: The primary function is to join separate network segments, which are usually LANs, into a unified network. This allows devices on different segments to communicate as if they were on the same network.
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Filtering and Forwarding Traffic: Bridges examine the MAC address of incoming packets. Based on the destination MAC address, the bridge either forwards the packet to the appropriate segment or filters it, preventing unnecessary traffic from crossing segments. This reduces network congestion.
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MAC Address Learning: Bridges learn the MAC addresses of devices connected to each segment. They build a MAC address table, which maps MAC addresses to specific network segments. This table enables the bridge to make intelligent forwarding decisions.
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Collision Domain Reduction: By segmenting the network, bridges reduce the size of collision domains. A collision domain is an area where two devices transmitting simultaneously can cause a collision, disrupting network communication.
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Protocol Transparency: Bridges operate at the data link layer (Layer 2) of the OSI model and are generally protocol-independent. This means they can forward traffic regardless of the higher-level protocols being used (e.g., TCP/IP).
How a Bridge Works:
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Learning: When a bridge is first powered on, its MAC address table is empty. As devices send traffic, the bridge records the source MAC address and the segment from which the traffic originated.
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Forwarding: When a packet arrives at the bridge, it examines the destination MAC address.
- If the destination MAC address is in the bridge's table and associated with a different segment, the bridge forwards the packet to that segment.
- If the destination MAC address is not in the table, the bridge floods the packet to all segments except the one it was received from. This ensures the packet reaches its destination, and the bridge learns the location of the destination device when it replies.
- If the destination MAC address is on the same segment as the source, the bridge filters the packet, preventing it from crossing to other segments.
Example Scenario:
Imagine two separate LANs in an office building. A bridge connects these two LANs. If a computer on LAN A wants to send a packet to a computer on LAN B, the bridge intercepts the packet, examines the destination MAC address, determines that the destination is on LAN B, and forwards the packet only to LAN B. Traffic destined only for devices on LAN A is not forwarded to LAN B, and vice versa.
Benefits of Using Bridges:
- Improved Network Performance: By reducing traffic congestion and collision domains.
- Increased Network Size: By connecting multiple LANs.
- Simple Implementation: Relatively easy to install and configure.
- Cost-Effective: Compared to more complex networking devices.
While bridges are useful, modern networks often utilize more sophisticated devices like switches, which offer enhanced features such as higher port densities, faster speeds, and advanced management capabilities. Switches can be thought of as multi-port bridges.
