Networks of workstations (NOW) are becoming a very popular alternative to parallel computers for those applications with high needs of resources such as memory capacity, processing power, and input/output storage space. Typically, these networks connect workstations using irregular topologies, providing the wiring flexibility, scalability, and incremental expansion capability required in this environment.In order to analyze and design these kinds of systems it is necessary to have adequate tools. To address this problem, we have implemented a very flexible and easy to use NOW simulator. It supports a technique for message fragmentation in packets, generates self-similar traffic, and it can model networks with permanent faulted links or switches.In this paper we present this NOW simulator, the basic queuing models it is based on, its main internal organization, input parameters, output performance variables, and finally, we show several simple examples of performance measures obtained for, among others, message fragmentation, failures in links and switches, and self-similar traffic.