Port Blocked

Double checked my web page, found that it can be reached by other PC within my router, but can't be reached from outside. Finally, found that I have to change the Port# from outside, and set my router to forward it (say 1025) to Port 80 of my server. May be the Port 80 is blocked by the ISP.

Now, my server can be reached from outside, with port 1024 specified.
Such as: http://XXX.XXX.XXX.XXX:1024/