The main purpose of a repeater is to extend the length of a network beyond its normal cable lengths. A repeater works like an amplifier to increase or boost the signal to allow transmissions over longer distances. Repeaters are used to connect network segments (groups of computers on the same network). They can also be used to connect segments composed of different media (for instance, a ThinNet segment to a fiber-optic segment).
Bridges work like repeaters, but offer additional advantages. They can isolate network traffic or problems. Should any problems occur within one segment, the bridge will isolate that segment and not affect other segments on the network, thereby reducing the load on the network as a whole. Bridges can also link segments that are unalike (such as Ethernet and token ring).
Routers provide interconnectivity between like and unlike devices on the LAN and WAN. Routers work like bridges, but can connect networks using different protocols. They are able to select the best route from one network to another network based on traffic load. Routers determine the flow of data based on such factors as least-cost, minimum delay, minimum distance, and least congestion. Routers are generally used to create a WAN and connect dissimilar networks.
Gateways provide all the connectivity of, and even greater functionality than, routers and bridges. A gateway usually resides on a dedicated computer that acts as a translator between two completely dissimilar systems or applications. Because gateways are both translators and routers, they tend to be slower than bridges or routers. Gateways also provide access to special services such as e-mail or fax functions.