I am trying to open ports 500 and 4500 with UDP Protocol to use a VPN application. My ISP configured the settings on my router and says the ports shoudl be open. However, the VPN is not working properly and when I use various port checkers to test whether the ports are open I get "timeout" error messages or, in the case of an application called PFPortChecker (from www.portforward.com) the messgae reads "could not test port 500 because some other application has that port locked. Please close any applications that may be using this port and try again."
I ran "netstat -o" in the cmd prompt to see what ports are being used by all active connections and there is nothing using either port 500 or 4500. The ISP agreed with me but say that since they think the router settings are correct they have no further interest or responsibility to help me actually solve the problem.
Does anybody please know a way to identify whether an application is "locking" a port, other than the cmd I have tried already?