Abstract: A simulation tool (ES) for a networking code (RE), characterized in that the code is carried in a computer (ORD) external to the system, the tool constituting an application for the operating system (NY) of the computer by being considered to be a virtual core (CC), and including at least the communications layer code, unmodified and compiled in the internal language of the computer (CC); the basic services for the virtual core, which are connected to said code (CONFIG, RA, SI, GH); a number (m) of libraries (L.sub.1 -L.sub.m) for access to the code (CC) for a number (n) of test applications (A.sub.1 -A.sub.n) of the latter code. The tool has particular utility in debugging networking codes.