An intranet-especially one in a large corporation-can be a remarkably complex endeavor, consisting of many networks, including departmental local area networks, as well as larger subnetworks, which each are in essence collections of networks.
Bridges connect networks. When bridges were first developed they could not connect incompatible networking architectures. Early bridges functioned at the media-access control (MAC) section of the data-link layer of the OSI model (discussed in Chapter 2. Ethernet and Token Ring architecture differ at the MAC section of the data-link layer, hence their incompatibility. Newer bridges function at the logical-link control (LLC) portion of the data-link layer. As long as the communication protocol on both networks is the same (IPX to IPX, for example), Ethernet and Token Ring networks can be linked by bridges. Connecting divergent networks is an essential requirement for intranets.
Bridges are also used to connect networks indirectly by long-distance, usually leased, lines. Remote bridges on Ethernet networks use a transparent routing technique to handle traffic. It is the destination address of each packet that is read to determine the action to be taken. If the destination is on the same network, bridges ignore the packet; bridges pass only packets that need to go to other networks. Token Ring operates somewhat differently in that it uses source routing. In this case, the bridge uses a test message to calculate the best path between the source and the destination.
In order to determine whether to pass or drop a packet, bridges refer to a table, which is basically a list of addresses. These tables originally had to be built manually, causing considerable maintenance every time a replacement or additional NIC address was put on the network. This led to the development of learning bridges, that is, bridges with software algorithms that could build these tables automatically. Today's bridges build and maintain their tables by listening to cable traffic and checking packets for source addresses.
Bridges are relatively simple to set up and maintain, and they can operate fairly fast since they simply decide whether to pass or drop the packet. In the next chapter a similar but more complex piece of equipment, the router, is discussed. Bridges are often used to segment and reduce local network traffic. Routers are usually used for connecting entire networks and subnetworks together.
Bridges and routers are sometimes combined into a single product called a brouter. A brouter combines the functions of both bridge and router. It examines the outermost address to see if the data needs to be sent to another LAN over a bridge, and delivers information that way if possible. If, instead, it needs to be sent using IP technology, it will act as a router does.
Bridges are hardware and software combinations that connect different parts of a single network, such as different sections of an intranet. They connect local area networks (LANs) to each other. They are generally not used, however, for connecting entire networks to each other, for example, for connecting an intranet to the Internet, or an intranet to an intranet, or to connect an entire subnetwork to an entire subnetwork. To do that, more sophisticated pieces of technology called routers are used. (See Chapter 4for more information on how routers work.)