Simulation is the main tool for studying networking protocols before deploying them in a wide scale, or for understanding how they are expected to behave under various conditions. IRLSim is a new packet level network simulator that we developed in the hope to study several Internet protocols. From its modest inception as a simulator for the RSVP signaling protocol, IRLSim has evolved into a more general purpose, easy to use, scalable simulator that can be used as a guide for studying existing network protocols as well as a research tool for developing new protocols. This paper describes the architecture of IRLSim in detail, presents its use in the study of a few specific networking problems and argues about its usefulness amongst the variety of other simulators.