Frequency scarcity has emerged as a primary hurdle when trying to launch new wireless communication services. However, recent spectrum utilization measurements have shown that the available spectrum opportunities are highly underutilized. The concept of Opportunistic Spectrum Access based on the technology of cognitive radio is one of the most promising approaches to help us understand and explore the opportunities presented by idle frequencies. In this thesis, a C++ based simulator for large-scale cognitive radio networks under an ad-hoc architecture was introduced and implemented. The simulator allows users to set the necessary parameters according to the Opportunistic Spectrum Access model, and distributes the primary nodes and secondary nodes according to a given distribution. In particular, we studied connectivity of large scale cognitive radio networks using this simulator. By using Breadth-First search algorithm, we were able to keep track of all components of the secondary network. MATLAB script is used to display the network topology using the data generated from the simulator. The simulator developed can serve as an invaluable tool for investigating the design and implementation of Cognitive Radio networks. Using the simulator to emulate and validate the network connectivity is one example presented in this thesis. We confirm our simulated results with analytical results.