Abstract: A development system providing a Custom Attack Simulation Language (CASL) for testing networks is described. In particular, the development system implements methodology for facilitating development of network attack simulations. The system includes an editor or authoring system for creating a source code description or Scripts (i.e., CASL-syntax Script) of the simulation program under development. The Scripts, in turn, are “compiled” by a CASL compiler into a compiled CASL program, that may then be used to simulate attacks against a network. CASL makes it easier for users, particularly network and system administrators, to experiment with and learn about the way their networks operate. Since networks work by exchanging packets of information, CASL focuses on allowing users to read and write packets directly to and from the network using a high level programming language.