What is a Network Bridge?

Vanessa Harvey

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 one 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 what is known as a Media Access Control (MAC) address, also called a physical or hardware address. It is this address that uniquely identifies a device to a bridge that can then determine to which network the device is connected.

Multiple networks systems can be connected by using a network bridge.
Multiple networks systems can be connected by using a network bridge.

The principal 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 what are known as collision domains. One way of defining a collision domain is a network in which one device, also called a node, forces every other device to listen when it is transmitting data packets. Another definition states that a collision domain exists when two or more devices attempt to transmit information at the exact 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.

A network hub.
A network hub.

Whenever collisions occur, the efficient transmission of data packets is greatly compromised. The more devices that are on a network trying to transmit data, the greater the chance for a collision to occur. A network bridge can be used to segment one network into two, thereby reducing the number of devices competing for transmission privileges. For example, if network A has 20 devices, there is the likelihood that two or more of them will attempt to transmit data at the same time and cause a collision. If a bridge is added, it can split network A into networks A and B of 10 devices each.

Once the network bridge is incorporated, it will begin to "listen" to the transmission of data performed by devices on the two networks. It accomplishes this by recording the MAC address of the devices 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 a forwarding table for future reference. The bridge also looks at the MAC address of the destination or receiving device. If it does not appear in its table, the bridge will broadcast the data packet to all devices on both networks to locate the intended destination.

Forwarding tables are not instantly built, rather the network bridge has to wait until it receives a transmission from a device before it can learn its MAC address. MAC addresses of receiving devices also have to be learned via broadcast, a search for the location of the destination. Once the destination responds, 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 straight to their destination. This will happen without all devices having to listen to one 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. The prevention of collisions is not the only advantage of using bridges; they also are 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 might maintain privacy of information.

A network bridge may be used to connect a single copier or printer to two separate networks.
A network bridge may be used to connect a single copier or printer to two separate networks.

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Discussion Comments


Layer 2 switch is no bridge. They serve the same function, but it's not the same. Both separate Collision Domains (CD), not networks. Devices are still in the same network. Here is what Cisco CCENT/CCNA cert book suggests: "The bridge, a predecessor to today's Ethernet LAN switch, uses logic so that frames in one CD won't collide with frames in other CD." All in all they shared same logic, but switch can perform the same task with much greater speeds (allows full-duplex), in addition it has many enhanced features. --Yuri M


@jacksbaxy -- A filter is a filter, not a bridge, it separates the 2 frequencies on which DSL data and voice data were being sent on, it filters them, it is not at all a bridge.


When we first got DSL, we could not use our landline phones if the DSL was plugged in. All you could hear was static. We had to buy a "filter" for the land line. I guess this was a network bridge of sorts. Sounds like the information being sent by the DSL and the land line where colliding, and the filter was acting as a bridge, allowing them to separate.


This is such an informative article. We have only recently began using a wireless network, and this explanation of a network bridge helped me understand better how a wireless network bridge works. Now I actually understand what is going one when we connect to the internet.

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