reach internal iis web server from outside sub domain

hi everyone 

i want to reach my internal iis server from outside using a sub-domain

i have used virtual ip adress to map my wan ip adress on my intenal adress server. it works.

But i wan to map the adress (http://ip public adress wan) to a sb domain , to mask my wan adress of my fortigate .

i would like to have (http:subdomain.net) to reach the same internal web server. I have redirect the subdomain to (http://ip public adress wan), but it don't work properly.

i need help