This paper presents pp-mess-sim, an object-oriented discrete-event simulation environment for evaluating multicomputer networks. The simulator provides a toolboz of various network topologies, communication workloads, routing-switching algorithms, and router models. These router models can vary from high-level architectures to low-level specification of actual devices. By decoupling individual parts of the code, pp-mess-sim enables independent code development and creates a flexible and extensible environment for evaluating different aspects of network design. Sample simulation experiments capitalize on this flezibility to compare routing-switching schemes under various application workloads.