bridges and network bridging

Shanghai bridge
xijian/Getty Images

A network bridge helps to join two otherwise separate computer networks together to enable communication between them. Bridge devices are used with local area networks (LANs) for extending their reach to cover larger physical areas.

How Network Bridges Work

Bridge devices inspect incoming network traffic and determine whether to forward or discard it according to its intended destination. An Ethernet bridge, for example, inspects each incoming Ethernet frame - including the source and destination MAC addresses, and sometimes the frame size - in making individual forwarding decisions.

 Bridge devices operate at the data link layer (Layer 2) of the OSI model

Types of Network Bridges

Several different kinds of bridge devices exist, each designed for specific kinds of networks including

  • Wireless bridges - support Wi-Fi wireless access points
  • Wi-Fi Ethernet bridges - allows connecting Ethernet clients and interfacing them to a local Wi-Fi network, useful for older network devices that lack Wi-Fi capability

Wireless Bridging

Bridging is especially popular on Wi-Fi computer networks. In Wi-Fi, wireless bridging requires access points communicate with each other in a special mode that supports the traffic needing to flow between them. Two access points that support wireless bridging mode work in pairs. Each continues to support their own local networks of connected clients while additionally communicating with the other to handle bridging traffic.

Network professionals sometimes use the term "BSS" - Basic Service Set - to refer to an access point and its local clients.

 Wireless bridging joins two BSS networks together.

Bridging mode can be activated on an access point through an administrative setting or sometimes a physical switch on the unit. Not all access points support wireless bridging mode; consult the manufacturer's documentation to determine whether a given model supports this feature.

For more, see: What Wireless Bridging Can Do For Computer Networks

Bridges vs. Repeaters

Bridge and network repeater devices share a similar physical appearance; sometimes, a single unit performs both functions. Unlike bridges, however, repeaters do not perform any traffic filtering and do not join two networks together but instead pass along all traffic they receive. Repeaters serve primarily to regenerate traffic signals so that a single network can reach longer physical distances.

Bridges vs. Switches and Routers

In wired computer networks, bridges serve a similar function as network switches. Traditional wired bridges support one incoming and one outgoing network connection (accessible through a hardware port), whereas switches usually offer four or more hardware ports. Switches are sometimes called multi-port bridges for this reason.

Likewise, bridges lack the intelligence of network routers: Bridges do not understand the concept of remote networks and cannot redirect messages to different locations dynamically but instead support only one outside interface.

See also: The Differences Between Routers, Switches and Hubs