In this work we present a prototype for simulating computer network attacks. Our objective is to simulate large networks (thousands of hosts, with applications and vulnerabilities) while remaining realistic from the attacker's point of view. The foundation for the simulator is a model of computer intrusions, based on the analysis of real world attacks. In particular we show how to interpret vulnerabilities and exploits as communication channels. This conceptual model gives a tool to describe the theater of operations, targets, actions and assets involved in multistep network attacks. We conclude with applications of the attack simulator.