List of unused ports on a machine
 

For a certain application, I create a socket at run time, that listens to the specified port. How do i know that which port numbers are available for use? I am not talking about 'legally' available, I just want to know the port numbers that are currently not being used on my system.

"nestat -an|grep -i listen" command will show u what ports listened currently. most of port numbers listed in /etc/services file.
http://www.iana.org/assignments/port-numbers