What Is a Network Bridge?

Learn how a Layer 2 network bridge connects LAN segments, separates collision domains, forwards Ethernet frames by MAC address, and compares with hubs and switches.

A network bridge is a Layer 2 device that connects two portions of an Ethernet local area network (LAN) while controlling which frames cross between them. It can keep local traffic on its own LAN segment and forward traffic to another segment only when necessary.

Bridging was especially useful in older shared-media Ethernet networks. Dividing one busy LAN into separate segments reduced unnecessary traffic and collisions. In modern Ethernet networks, switches usually perform the same basic function with multiple ports.

What Is a Network Bridge?

A LAN segment is a portion of a local network connected through a bridging device. A bridge joins two LAN segments, but it does not treat every frame as traffic that must cross from one side to the other.

The bridge receives an Ethernet frame, examines its destination MAC address, and decides whether to:

Forward the frame to the other segment because the destination device is there.
Filter the frame by keeping it on the current segment because the destination device is local to that segment.

This selective behavior limits unnecessary local Ethernet traffic. If many conversations stay within their own segments, each segment can use its available transmission capacity without every frame occupying the other segment.

A Bridge at the OSI Data Link Layer

A bridge operates primarily at OSI Layer 2, the Data Link layer. This layer handles Ethernet frames and uses MAC addresses to make local forwarding decisions. A MAC address is a Layer 2 hardware address assigned to a network interface.

It is useful to distinguish a bridge from devices at other layers:

Layer 1 hub: Repeats electrical or signal activity to its ports. It does not inspect destination MAC addresses or make forwarding choices.
Layer 2 bridge: Examines Ethernet frames and controls whether frames cross between connected segments.
Layer 3 router: Examines network-layer information, such as IP addresses, and forwards packets between different IP networks.

For background on how the layers relate to one another, see the OSI Reference Model.

Collision Domains and Bandwidth

A collision domain is a shared network area in which simultaneous Ethernet transmissions could collide. This concept is most important for legacy shared Ethernet, where devices competed for access to the same medium.

Without a bridge, two connected groups using one shared medium may belong to one collision domain. A bridge separates them into two collision domains:

Devices on the first segment contend with one another on that segment.
Devices on the second segment contend with one another on that segment.
Traffic that stays on one segment does not consume the shared medium of the other segment.

Each bridged segment has its own available bandwidth rather than sharing one collision domain with the other segment. This does not automatically increase the physical speed of either segment, but it allows the segments to carry traffic independently. In a busy legacy LAN, fewer collisions and more localized traffic can improve overall efficiency.

How a Bridge Handles Ethernet Frames

When a frame arrives, the bridge examines the frame's destination MAC address and determines where the destination is located.

Source Host Segment	Destination Host Segment	Destination MAC Address Examined	Bridge Action	Result
One segment	Same segment	Destination MAC belongs to a device on the incoming segment	Filter the frame; do not send it to the other segment	Local traffic remains on its original segment
One segment	Other segment	Destination MAC belongs to a device on the opposite segment	Forward the frame through the bridge	The destination segment receives the frame

The forwarding or filtering decision determines which segment receives the frame. A frame that is filtered does not need to consume capacity on the other side. A frame that is forwarded is transmitted onto the segment where the destination device is located.

Example: Four Hosts on Two Bridged Segments

Consider a small LAN with four hosts. Hosts A and B share one Ethernet segment. Hosts C and D share a second Ethernet segment. A bridge connects the two segments.

Collision Domain 1                 Collision Domain 2
+-------------------+              +-------------------+
| Host A   Host B   |              | Host C   Host D   |
+---------+---------+              +---------+---------+
          |                                  |
          +---------- [ Bridge ] ------------+

Each side of the bridge is a separate collision domain. The bridge does not merge the two sides into one shared collision area.

Host A Sends to Host C

Host A creates an Ethernet frame with Host C's MAC address as the destination MAC address.
The frame travels across Host A's segment and reaches the bridge.
The bridge examines the destination MAC address in the Ethernet frame.
The bridge determines that Host C is located on the second segment.
The bridge forwards the frame onto the second segment.
Host C receives the frame because its MAC address matches the destination address.

Host A  -- Ethernet frame: destination = Host C MAC -->  Bridge
                                                       inspect MAC
Host C segment  <-- forwarded frame -------------------+
                                                       Host C receives

The bridge does not forward the frame blindly to both sides. It uses the Layer 2 destination address to control where the frame goes.

Host A Sends to Host B

Now suppose Host A sends a frame to Host B. Both hosts are on the first segment. The bridge identifies Host B's MAC address as local to that segment and filters the frame instead of sending it to the second segment.

This filtering keeps the A-to-B conversation from consuming transmission capacity on the segment containing Hosts C and D. Localized traffic is one of the main efficiency benefits of bridging.

Bridge Compared with a Hub

A network hub is a Layer 1 device. It repeats incoming signals out of its other ports without making a MAC-address-based forwarding decision. All devices attached through a traditional hub share one collision domain.

A bridge is more selective. It inspects Ethernet frames and can keep local traffic on one side while forwarding traffic for the opposite side. Therefore, replacing one shared hub arrangement with two bridged segments creates separate collision domains and reduces competition for the same shared medium.

Device	Primary OSI Layer	Forwarding Basis	Collision Domain Behavior	Typical Modern Use
Hub	Layer 1, Physical	Repeats signals; no MAC-based decision	All attached devices share one collision domain	Rare in current Ethernet networks
Bridge	Layer 2, Data Link	Destination MAC address in the Ethernet frame	Creates separate collision domains for connected segments	Standalone bridges are uncommon
Switch	Layer 2, Data Link	Destination MAC address and a MAC-address forwarding table	Each switch port normally forms a separate collision domain	Standard device for most modern Ethernet LANs

Bridge Compared with a Switch

An Ethernet switch is commonly described as a multiport Layer 2 bridge. It performs the same fundamental type of work: receiving Ethernet frames, examining MAC addresses, and forwarding or filtering frames between ports.

A traditional standalone bridge generally connects a small number of LAN segments. A switch provides many ports, allowing it to connect numerous hosts and network devices while applying Layer 2 forwarding decisions independently at each port.

Standalone network bridges are uncommon in modern LAN deployments because switches normally provide the relevant bridging function more efficiently and at greater scale. When learning the fundamentals, however, understanding a two-port bridge makes the basic logic of Ethernet switching easier to follow.

For a broader introduction to networks, review Computer Network Expained.

Common Misunderstandings and Troubleshooting

Devices on Separate Bridged Segments Cannot Communicate

Check the physical connectivity between the bridge and each LAN segment. Confirm that the destination device is connected and functioning. Then verify the basic forwarding concept: the frame must reach the bridge, the bridge must inspect the destination MAC address, and the frame must be transmitted toward the destination segment.

A Bridge and a Hub Provide the Same Segmentation

They do not. A hub repeats traffic and leaves attached devices in one collision domain. A bridge divides collision domains and can filter traffic that is destined for a device on the same segment.

A Bridge Is the Normal Device for a Current Ethernet LAN

Modern Ethernet LANs generally use switches. A switch supplies the essential Layer 2 bridging behavior across multiple ports, so a separate two-port bridge is rarely needed in an ordinary current deployment.

Key Points to Remember

A network bridge connects and separates LAN segments at OSI Layer 2.
It examines Ethernet frames and uses destination MAC addresses for forwarding decisions.
Forwarding sends a frame to the other segment when the destination is there.
Filtering keeps a frame local when the destination is on the incoming segment.
A bridge creates separate collision domains, allowing the segments to use their available bandwidth independently.
A hub is a Layer 1 repeater and does not make MAC-based forwarding choices.
A switch is essentially a multiport Layer 2 bridge and is the usual modern replacement for a standalone bridge.