Class auth not working

[eluser]novarli[/eluser]
Class auth not working

I have web application using CI. If i use domain in

$config['base_url'] = "http://www.domain.com";

But when i change

$config['base_url'] = "http://123.456.1.2/folder";

User cannot login. If username and password inccorect will appear error message.
But if both correct cannot redirect to main page. But still at login page with not display error message. But if i use domain no problem.

Why?

Thanks before ...