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The simplest way to interconnect LANs is to use a hub. A hub is a simple device that takes an input (i.e., a frame's bits) an retransmits the input on the hub's outgoing ports. Hubs are essentially repeaters, operating on bits. They are thus physical-layer devices. When a bit comes into a hub interface, the hub simply broadcasts the bit on all the other interfaces. In this section we investigate bridges, which are another type of interconnection device.
In contrast to hubs, which are physical-level devices, bridges operate on Ethernet frames and thus are layer-2 devices. In fact, bridges are full-fledged packet switches that forward and filter frames using the LAN destination addresses. When a frame comes into a bridge interface, the bridge does not just copy the frame onto all of the other interfaces. Instead, the bridge examines the destination address of the frame and attempts to forward the frame on the interface that leads to the destination.
Up until the mid 1990s, three types of LAN interconnection devices were essentially available: hubs (and their cousins, repeaters), bridges and routers. More recently yet another interconnection device became widely available, namely, Ethernet switches. Ethernet switches, often trumpeted by network equipment manufacturers with great fanfare, are in essence high-performance multi-interface bridges. As do bridges, they forward and filter frames using LAN destination addresses, and they automatically build routing tables using the source addresses in the traversing frames. The most important difference between a bridge and switch is that bridges usually have a small number of interfaces (i.e., 2-4), whereas switches may have dozens of interfaces. A large number interfaces generates a high aggregate forwarding rate through the switch fabric, therefore necessitating a high-performance design (especially for 100 Mbps and 1 Gbps interfaces).