A router operates through the first three layers of the OSI model, which adds the network layer to its range of known information.

Bridges can find devices in a connected network because device addresses are carried in layer two, which is also called the MAC (media access control) layer. Bridges cannot find specific network protocol addresses (such as IP or subnet addresses; or IPX or Appletalk addresses) because that information is carried in layer three.

Routers can find discreet network addresses when they are configured to work with that particular protocol. Configuring a router to work with a protocol requires software for each protocol that the router must negotiate.

Routers learn optimum paths to network destinations by keeping up-to-date information in routing tables.

When should you bridge or route? -- You bridge when you are connecting networks of the same type (such as all IP, or all IPX). You route when you are connecting different types of networks (such as connecting and IP network to an IPX network). From a practical point of view, you should always route if possible, as routing is more efficient and makes call management easier. Bridging is necessary when you cannot subnet your IP network, and when you need to use non-routable protocols such as NetBIOS, or DECnet.

Overall, bridging is very easy to configure and allows you to connect to a network transparently - it looks as if you are directly connected. However, there is a penalty in both performance and call management. Here is why: the first packet from each computer is automatically forwarded - this is how bridges learn which address is on which side of a link. The bridge address table is automatically built using this method. Since broadcast packets are addressed to all computers on the network, they are always forwarded when bridging, thus impacting performance and causing unnecessary calls to be placed.

Alternatively, routing operates on layer three of the OSI model, which is the network layer. Routers can make more intelligent decisions about sending a packet across the network by examining the source and destination address of each packet. It only forwards packets destined for a remote network. This eliminates unnecessary traffic and makes better use of bandwidth. It also improves call management.