The Internet supports migration of code from node to node. A number of paradigms exist for distributed computing and mobile code, including client/server, remote evaluation, code-on-demand, and mobile agents. We find them overly-restrictive views of reality. We propose a model that can express previous paradigms as special cases. We derive a model using cellular automata (CA) abstractions to study relations between local node behavior and global system behavior. Example mobile code systems are provided and existing paradigms are expressed in terms of the model. These examples include network attacks such as viruses, worms, and distributed denial of service (DDoS). A distributed system simulation tool based on the model is described. Simulation results provide insights gained from this work.