How to check whether a port is open at client's network/firewall?

Question

This is solved at last with \"timeout\" attribute of jQuery AJAX (and JSONP). See my own answer !

Please see the updated part, I

Answer 1

Gotcha !!! I have solved my problem with JSONP and jQuery AJAX call. I discovered the timeout attribute of jQuery AJAX and my code executed fluently when the port was blocked or opened. Here is the solution for future visitors. Thanks to all answerers for contribution.