How can I tell which ports are available to use? Is there a list of “official” port numbers?
Usually this isn’t an issue. If you’re writing, say, a web server, then it’s a good idea to use the well-known port 80 for your software. If you’re writing just your own specialized server, then choose a port at random (but greater than 1023) and give it a try. If the port is already in use, you’ll get an “Address already in use” error when you try to bind(). Choose another port. (It’s a good idea to allow the user of your software to specify an alternate port either with a config file or a command line switch.) There is a list of official port numbers maintained by the Internet Assigned Numbers Authority (IANA). Just because something (over 1023) is in that list doesn’t mean you can’t use the port. For instance, Id Software’s DOOM uses the same port as “mdqs”, whatever that is. All that matters is that no one else on the same machine is using that port when you want to use it.
