A system for testing communication software, such as a network stack that may be used in a personal computer or other computing device. The system includes software to emulate the functionality of hardware of two or more computers. This software may be executed on a single computer, thereby simplifying testing or providing greater control over the test environment by avoiding unintended interactions with hardware connected over a physical network connection. The emulation software includes interfaces that emulate the interfaces used in hardware drivers for network cards in the computers. These interfaces allow the emulation software to interface to the same test applications and tools that would be used in a hardware test so that testing using emulation software may be used instead of or in conjunction with testing using hardware. The emulation software is customizable through the use of modular components, including a component through which user provided code may be readily integrated into the emulation software.