What is the range of valid ports for a ServerSocket to listen to?

You can create a server on ports 1 through 65535. Port numbers less than 256 are reserved for well-known services (like HTTP on port 80) and port numbers less than 1024 require root access on UNIX systems. Specifying a port of 0 in the ServerSocket constructor results in the server listening on a random, unused port, usually >= 1024.