The routing component allows you to calculate shortest and fastest routes. To do so, you require a shapefile that contains topologically correct links. If you have a shapefile containing roads, for example, the roads should be captured from intersection to intersection, and the roads' start and end points must coincide exactly with the start and end points of the roads with which they are joined. You can obtain topologically correct data from commercial vendors, such as those who supply address data. Here is one example of such a data vendor that supplies topologically-correct network and address data.

Each road segment is known as a link. Each link has a length and a cost. Typically the cost is the travel time. Links are directional. For example, a one-way street allows travel in one direction but not in the other direction. The cost in the two possible directions of travel may differ. A road may have two lanes in one direction but only one lane in the other direction. The travel cost would be higher in the direction with only one lane. 

The costs and other link attributes such as its name are read from the DBase file accompanying the shapefile. The DBF file for example could have field called traveltime12 and traveltime21, which could be numbers containing the costs for travel in a direction from start to end and end to start respectively. When setting up a network layer, you have to specify which fields in the DBase file contain which link information.