I’m not sure what is happening there. Networking.h includes IPv4SubnetMask.h as well, so I am not sure why you are seeing errors relating to definition for that class. I tried to reproduce those errors, and was unable to do so. Do you think you might be able to create a new project from one of templates and add in some of your networking code until this error starts to appear? It looks like there is a problem with a header file somewhere, but I can’t track it down without being able to reproduce it.