Neon-komputadór

Bridges, Routers and Brouters

Bridges are used to connect multiple local area networks that use the same addressing system. They are commonly used on local area networks as an inexpensive means to filter traffic and therefore increase the number of clients on the network. They can split overcrowded segments, expand the number of nodes in a network and, in some cases, link different typologies. By building a table of physical addresses to the clients connected to a local area network and transmitted to the bridge, it can filter packets either to the local node or to another local area network. When data is transmitted on one of the network segments, the bridge checks the destination address. If it is on the same network segment, the bridge ignores it. If it is on a different segment the bridge forwards the data to the appropriate segment. Because bridges use the physical, rather than the logical, address they can't usually be used for connecting different network types. However, translating bridges provide a way for different local area networks (e.g., Ethernet and Token Ring) to communicate by converting packets from one format to another.

Routers are essential for any large network based on TCP/IP, Without routers there would be no Internet - or rather, the Internet would be very small, not really an Internet at all. Unlike a bridge, a router filters traffic by logical address and includes redundant pathways from the data source to the data destination. Also, significantly more sophisticated than bridges, many routers maintain information describing the best path based on distance, bandwidth and times. such as the Routing Information Protocol or the more recent Open Shortest Path First protocol. The two primary types of routing are static and dynamic. In the former, the network administrator must manually enter the route information into the table. In the latter, the table is built on the information received by routing protocols. Whilst the former is useful in some circumstances, obviously the latter is preferable and indeed becomes more necessary as the network expands.

A brouter or bridging router, as the name implies, is a device that can act as a bridge and a router and uses both the physical network address and the logical address. This is necessary for some protocol suites which are not routable, for example, Microsoft's NetBEUI protocol which is fairly common, particularly in small networks. A brouter routes routable protocols (such as TCP/IP) and bridges non-routable protocols (such as NetBEUI). Note that sometimes a network administrator may not want to use a brouter. For security and traffic reasons sometimes it is preferable to isolate a number of machines on a local area network.

A common way to distinguish between all these various connectivity devices is to think of them in terms of layers. Bridges for example are link-level connectors. They forward frames from one link to extend a local area network. Switches are network level connectors, as they forward from one link to another to implement a packet-switched network. Routers are internet-level connectors, as they forward datagrams from one network to another to develop an internet. It's not a precise definition (as some companies will make connectors that transcend these distinctions), but it's a common working model. The most precise method is to distinguish between the various devices is that repeaters do nothing but repeat a signal, switches and routers operate on routing protocols that learn the typology of the whole network and bridges utilize a spanning tree algorithm.