Redirect no-ip.com and Log in problem

I have installed Drupal 4.6.5 on my PC . Now the site could be seen from "outside" fine, and the users could login .
But i don't have static ip or domain name so i used no-ip.com to use a domain xxx.no-ip.org .
I changed the base_url in settings.php but now the users now can't log in .

Does anyone have a solution fo this ?