Networking: A Beginner's Guide
Because they operate below the network layer at which protocols such as TCP/IP and IPX/
SPX are defined, bridges don't care about the network protocols they're carrying. They care only
about the information required to operate at the data-link layer. This means that whether or not data
is carried over the bridge depends on its MAC address.
You should use bridges only on smaller networks, or in cases where you would
otherwise use a repeater, but would benefit from keeping traffic on one segment from
being transmitted on the other segment unnecessarily. Often, routers or switches
offer solutions that perform better and create fewer problems, so examine these other
options before choosing a bridge.
Just as bridges are basically more intelligent repeaters, routers are more intelligent
bridges. Routers operate at the network layer (layer 3) of the OSI model, and they are
far more intelligent than bridges in sending incoming packets off to their destination.
Because routers operate at the network layer, a connection across a router requires only
that the higher layers use the same protocols. The router can translate from any of the
protocols at layers 1 through 3 to any other protocols at layers 1 through 3 (provided
the router has been configured and designed to do so). Routers can connect both similar
and dissimilar networks. They are often used for wide area network (WAN) links.
Routers actually become a node on a network, and they have their own network
address. Other nodes send packets to the router, which then examines the contents of
the packets and forwards them appropriately. For this reason, routers often have fast
microprocessors--usually of the reduced instruction set computer (RISC) type--and
memory built into them to perform this job. Routers can also determine the shortest
route to a destination and use it. They can perform other tricks to maximize network
bandwidth and dynamically adjust to changing problems or traffic patterns on a
To learn about the networks to which they're connected, and what they should do to route
various types of packets properly, routers use a process called discovery. During the discovery
process, the router carefully "listens" to traffic on its ports and also sends out advertisement packets
letting other devices know of the router's presence.
Routers form the backbone of the Internet. When you use the TRACERT command
to trace the route from a node to a destination, most of the addresses that appear for the
hops are actually different routers, each one forwarding the packet to the next until it
reaches its destination.
Routers can route only protocols that are routable. AppleTalk, NetBIOS, and NetBEUI are
examples of protocols that are not routable, while TCP/IP and IPX/SPX are routable.