We present an open-source event-driven simulation package called QuISP built on top of the OMNeT++ discrete event simulation framework. Behavior of quantum devices in small quantum networks has been the focus of recent studies, it is still an open question how they will work in networks of practical size. QuISP is designed to simulate large-scale quantum networks to investigate their behavior under realistic, noisy and heterogeneous configurations. Our confidence in the simulator is supported by comparing its output to analytic results for a small network. QuISP can handle thousands of qubits in hundreds of nodes on a laptop computer, preparing for full Quantum Internet simulation. This simulator promotes the development of protocols for larger and complex quantum networks.