In this paper, we present Netzob, an opensource tool which supports the expert in its operations of reverse engineering, evaluation and simulation of communication protocols. Its main goals are to help security evaluators to assess the robustness of proprietary or unknown protocols implementation, simulate realistic communications to test third-party products (IDS, firewalls, etc.) or even create an open source implementation of a proprietary or unknown protocol. Netzob supports the expert in a semi-automatic manner. It includes the necessaries to passively learn the vocabulary of a protocol and to actively infer its grammar. In addition, it integrates a stochastic and statefull model to represent any statefull communication protocol. The definition of the model can be shared and loaded in a dedicated component of Netzob, its simulator. Therefore, it becomes easy to simulate multiple actors (servers and clients) which communicate according to the infered protocol, and to apply advanced fuzzing.