A distributed system always runs on top of a computer network and cannot be separated from it. In many cases this network consists of hundreds or thousands of computers and processing nodes. An effective distributed system simulator needs to simulate the underlying network. Unfortunately a great majority of existing simulation tools are pure network simulators. Even though they are very effective for designing, and evaluating computer networks, they could not be used to simulate a distributed application like a web service based application. Many components in a distributed system are complex servers and software applications running on top of all network layers. Network simulators cannot simulate them. A higher-level simulator is required to simulate their behavior. This work introduces an agent-based simulation model that integrates the simulation of a computer network and higher-level components of a distributed application. The distributed nature of agents makes them suitable to model and simulate distributed architectures including computer networks and distributed systems. To evaluate this approach the behavior of a replicated web service application will be simulated to show how effectively multi- agent-systems could be used to simulate the behavior of a distributed system.