This book is designed to be an NS2 user manual. It provides basic information required to install NS2, run simple examples and create as well as incorporate network modules with scenarios in NS2. To this end, the details of several built-in NS2 modules are explained in a comprehensive manner. NS2 by itself contains numerous modules. As time elapses, researchers keep developing new NS2 modules. This book does not include the details of incorporating new NS2 modules. We believe that once the basics of NS2 are grasped, the readers can go through other documentations, and readily understand the details of other NS2 components. The details of Network AniMator (NAM) and Xgraph are also briefed here. We understand that these two tools are nice to have and could help user to do simulation and analysis of computer networks. On the other hand, we believe that they are not essential to the understanding of the NS2 concept.