Bridging is a forwarding technique used in packet-switched computer networks. This article describes routing in computer networks, a method of finding paths from origins to destinations, along which information can be passed. ... In computer networking and telecommunications, packet switching is a communications paradigm in which packets (messages or fragments of messages) are individually routed between nodes, with no previously established communication path. ... A computer network is a system for communication between computers. ...

Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on broadcasting to locate unknown devices. Once a device has been located, its location is recorded in some fashion so as to preclude the need for further broadcasting. This article discusses routing in computer networks. ... Broadcasting in a computer network refers to transmiting a packet that will be received (conceptionally) by every device on the network. ...

The utility of bridging is limited by its dependence on broadcasting, and is thus only used in LANs. Currently, two different bridging technologies are in widespread use. Transparent bridging predominates in Ethernet networks; while source routing is used in Token-Ring networks. A local area network (LAN) is a computer network covering a small local area, like a home, office, or small group of buildings such as a college. ... It has been suggested that IEEE 802. ... Token-Ring local area network (LAN) technology was developed and promoted by IBM in the early 1980s and standardised as IEEE 802. ...

Transparent Bridging

Transparent bridging refers to a form of bridging "transparent" to the end systems using it, in the sense that the end systems operate in exactly the same way as in a non-bridged network. It is used primarily in Ethernet networks, where it has been standardized as IEEE 802.1D. Links ANSI/IEEE Std 802. ...

The bridging functions are confined to network bridges which interconnect the network segments. First, the network bridges use the spanning tree algorithm to build a loop-free network topology by selectively disabling network segments. This allows broadcasting to occur simply by copying packets; the tree structure insures that loops will not occur and that broadcast packets will therefore not be copied indefinitely. Next, the bridges monitor all packets traveling on the network, noting their source addresses, matching these addresses to the interfaces the packets were received on, and recording this information in a table. Finally, the bridges forward packets according to a simple algorithm - if the destination address does not appear in the table, broadcast the packet throughout the network; otherwise, forward it only on the interface recorded in this table. A network bridge connects multiple network segments (network domains) along the data link layer. ... A network segment is a portion of a computer network separated by a computer networking device, including: Repeater or hub (both OSI layer 1 devices) Bridge or switch (OSI layer 2 devices) Router (OSI layer 3 device) A bridge separates collision domains while a router separates both collision domains and... The spanning tree network protocol provides a loop free topology for any bridged LAN. The Spanning Tree Protocol, which is also referred to as STP, is defined in the IEEE Standard 802. ... Broadcasting in a computer network refers to transmiting a packet that will be received (conceptionally) by every device on the network. ...

Note that both source and destination addresses are used in this algorithm. Source addresses are used to record entries in the table, while destination addresses are used to lookup entries.

As a simple example, consider a packet exchange between two hosts (A and B) in a bridged network. A begins by transmitting a packet to B. Since B (the destination) does not appear in any of the bridges' tables, the packet is broadcast throughout the network. Each network bridge sees a copy of the packet, and records its source address (A) along with the interface it was received on. Each bridge now has an entry for A in its table. B, having received a packet from A, now transmits a packet in response. Since the bridges all have an entry for A (the packet's destination), no broadcast is required, and the packet is forwarded along the tree from B to A. All the bridges along this path record the packet's source address - B. Two-way communication is now possible between A and B without any further broadcasting. Note, however, that only the bridges along the direct path between A and B possess table entries for B. If a third host (C), not along the direct path, now transmits a packet for B, it will be broadcast throughout the network.

Source Route Bridging

Source route bridging is used primarily on Token-Ring networks, and is standardized in Section 9 of the IEEE 802.2 standard. The spanning tree algorithm is not used, the operation of the network bridges is simpler, and much of the bridging functions are performed by the end systems, particularly the sources, giving rise to its name. Token-Ring local area network (LAN) technology was developed and promoted by IBM in the early 1980s and standardised as IEEE 802. ... IEEE 802. ... The spanning tree network protocol provides a loop free topology for any bridged LAN. The Spanning Tree Protocol, which is also referred to as STP, is defined in the IEEE Standard 802. ... A network bridge connects multiple network segments (network domains) along the data link layer. ...

A field in the Token-Ring header, the Routing Information Field (RIF), is used to support source-route bridging. Upon sending a packet, a host attaches a RIF to the packet indicating the series of bridges and network segments to be used for delivering the packet to its destination. The bridges merely follow the list given in the RIF - if a given bridge is next in the list, it forwards the packet, otherwise it ignores it.

When a host wishes to send a packet to a destination for the first time, it needs to determine an appropriate RIF. A special type of broadcast packet is used, which instructs the network bridges to append their bridge number and network segment number to each packet as it is forwarded. Loops are avoided by requiring each bridge to ignore packets which already contain its bridge number in the RIF field. At the destination, these broadcast packets are modified to be standard unicast packets and returned to the source along the reverse path listed in the RIF. Thus, for each route discovery packet broadcast, the source receives back a set of packets, once for each possible path through the network to the destination. It is then up to the source to choice one of these paths (probably the shortest one) for further communications with the destination.

Source Routing Transparent (SRT) Bridging

Source routing transparent (SRT) bridging is a hybrid of source routing and transparent bridging, standardized in Section 9 of the IEEE 802.2 standard. It allows source routing and transparent bridging to coexist on the same bridged network by using source routing with hosts than support it and transparent bridging otherwise. IEEE 802. ...

General Context

In a more general context, bridging allows you to connect, for example; two different networks, a wifi and a ethernet network both on a laptop.