Available with Network Analyst license.
When authoring a geoprocessing tool that will be published as a geoprocessing service for solving network analysis layers, you will add one of six geoprocessing tools to your model or script: Generate Service Areas, Find Routes, Find Closest Facilities, Generate Origin Destination Cost Matrix, Solve Location-Allocation, or Solve Vehicle Routing Problem.
The Geoprocessing service example: Drive-time polygons tutorial and its companion data demonstrate how to create a tool for generating service areas in ArcGIS Server. The tutorial includes sample data for setting up services to solve vehicle routing problems and find closest facilities. Tutorial topics specific to other types of analysis aren't provided because the workflows for configuring other types of services are similar to setting up a service area service, so this tutorial is useful for other service types as well.