Massively multiplayer online games (MMOGs) have become highly popular in the last decade and now attract millions of users from all over the world to play in an evolving virtual world concurrently over the Internet. The high popularity of MMOGs created a rapidly growing market and this highly dynamic market has forced the game developers to step up competitively. However, MMOG development is a challenging and expensive process. In this study, we have developed a network simulation tool which can be used to model and simulate typical MMOGs that have client-server architectures. The main objective is to provide a simulation environment to MMOG developers that could be used to test, analyze and verify various aspects of the MMOG network architecture. We have also implemented a graphical user interface which allows constructing the simulation model visually. We have demonstrated the use of simulation tool by experimental simulations.