The Internet of Things (IoT) is comprised of networks of physical, computational, and human components that coordinate to fulfill time-sensitive functions in a shared operating environment. Development and testing of IoT systems often utilizes modeling and simulation, whether to analyze potential performance gains of new technologies or develop robust digital twins to support future operations and maintenance. However, the complexity and scale of IoT means that individual simulators are often inadequate to simulate the real-world dynamics of such systems, and simulators must be combined with other software or hardware.The National Institute of Standards and Technology (NIST) has developed a software module that extends the ns-3 network simulator with a new capability to communicate with external software and hardware at runtime. This software facilitates the development of co-simulations where ns-3 models can synchronize and exchange data with external processes to develop higher-fidelity simulations. The software is open-source and available on the NIST GitHub.