There's a set of ports allocated for http by the standards. The app will choose to use the first one available, starting with 80. If all such ports are in use then tell the user sorry all ports are already in use. Please stop some apps already running on your PC which may be using these ports.
So you have to run your app as root on Linux and windows firewall is going to have a field day? I don’t know what happens on a Mac, but I assume it will also require root to use low ports. Why not use port numbers dedicated to this kind of thing (1024 – 65535) or better yet, just use pipes and avoid opening a network server at all.
