Android : How do I know, from the smartphone, witch is the available port at the server?

I'm trying to make a client/server app for my final assignment at school. What I'm trying to do is to connect my tablet/phone to my computer and send an audio file that the computer will analyse and then it will light up some led's and stuff.

My problem is: I know how to connect a client to a server (with fixed ports) throw the sockets, but I would like to know wich port is the server using, insted of putting a fixed port on the code(at the client).

Can anyone help me?