Simulation is perhaps the most cost-effective tool to evaluate the operation of a system under design. A flexible, easy to extend, fully object-oriented, and multilayered simulator for interconnection networks can be a very useful tool for multicomputer designers and researchers. It is so desirable to attach newly designed components to the existing models and to exploit detailed results. This paper presents XMulator, an object-oriented listener-based simulation environment for evaluating multicomputer interconnection networks. The simulator involves a toolbox of various network topologies, routing algorithms, switching techniques, and flexible router models. This work introduces a simulator, using listener-based integration methodology, which has a great impact on extensibility of the system. Mixed-mode event processing improves the performance of the simulator. By decoupling individual parts of the code, XMulator enables independent code development and creates a flexible and extensible environment for different aspects of network design. This simulator uses XML format to define network topologies, input parameters, and outputs reports providing a high level of flexibility. To the best of the authors' knowledge, it is the first simulator enabled to simulate any arbitrary interconnection topology under different working conditions including in the presence of faults.