Stochastic extensions to Petri nets have received growing attention during the past decade as a model for evaluating the performance, dependability, and performability of computer hardware, software, and networks. Their formal structure permits solution by analytic means in many cases. When this is not possible, they an facilitate the automatic generation of a simulation program to estimate system behavior. The paper describes an X-window based software tool for evaluating systems that are represented as stochastic activity networks, a variant of stochastic Petri nets. The tool, known as UltraSAN, incorporates the results of recent research to significantly reduce the size of the state space that is considered for analytic solution, as well as the number of event types that are considered in simulation. Throughout the paper, a simple local area network model is used to illustrate the concepts, user interface, and model construction and solution methods implemented in the package.