edVirtus Reference

What Is a Network Bridge?

How Does a Bridge Connect Network Segments?

A network bridge is a data link layer device that connects two or more segments of a local area network (LAN). Operating at Layer 2 of the OSI reference model, a bridge examines the destination MAC address contained in each Ethernet frame and selectively forwards the frame only when it needs to cross from one network segment to another. This reduces unnecessary traffic and improves network efficiency.

The basic principle is straightforward. As Ethernet frames arrive at the bridge, it records the source MAC address associated with the receiving port in a forwarding table (sometimes called a bridge table or MAC address table). Over time, the bridge learns which devices are connected to each port. When a frame is received, the bridge consults this table. If the destination device is located on the same segment as the sender, the frame is discarded because forwarding is unnecessary. If the destination lies on another segment, the bridge forwards the frame only to the appropriate port. If the destination address is unknown, the bridge temporarily floods the frame to all ports except the one on which it arrived.

A useful analogy is a receptionist directing visitors within a building. Rather than sending every visitor through every corridor, the receptionist asks whom they wish to see and directs them only to the appropriate office. Similarly, a bridge forwards network traffic only where it is needed.

Unlike hubs, bridges divide a LAN into separate collision domains, reducing the number of devices competing for access to the same communication medium and improving overall performance. However, all connected segments remain part of the same broadcast domain, so broadcast frames are still forwarded throughout the bridged network.

As Ethernet technology evolved, the functions of a bridge became incorporated into the Ethernet switch. A modern switch is essentially a high-speed multiport bridge that performs the same Layer 2 forwarding function while supporting many more ports, higher throughput, full-duplex operation, and additional capabilities such as Virtual Local Area Networks (VLANs), Quality of Service (QoS), and link aggregation.

It is important to distinguish a bridge from a hub. A hub simply repeats every incoming signal to every connected device and has no knowledge of MAC addresses. A bridge examines MAC addresses and forwards traffic selectively, making communication more efficient. Likewise, a bridge differs from a router, which operates at the network layer (Layer 3) and forwards IP packets between different networks using routing tables rather than MAC addresses.

Today, standalone network bridges are relatively uncommon because Ethernet switches provide the same functionality with greater performance and flexibility. Nevertheless, the concept of bridging remains fundamental to Ethernet networking, and many modern switches continue to implement the learning and forwarding techniques originally developed for network bridges.