Discrete event simulation is widely used within the networking community for purposes such as demonstrating the validity of network protocols and architectures. Depending on the level of detail modeled within the simulation, the running time and memory requirements can be excessive.The goal of our research is to develop and demonstrate a practical, scalable approach to parallel and distributed simulation that will enable widespread reuse of sequential network simulation models and software. We focus on an approach to parallelization where an existing network simulator is used to build models of sub-networks that are composed to create simulations of larger networks.Changes to the original simulator are minimized, enabling the parallel simulator to easily track enhancements to the sequential version. In this paper we describe our lessons learned in applying this approach to the publicly available ns software package, and converting it to run in a parallel fashion on a network of workstations.This activity highlights a number of important problems, from the standpoint of how to parallelize an existing serial simulation model and achieving acceptable parallel performance.