Changed base_url - now sessions dont work

[eluser]Stuart Marsh[/eluser]
Hi all,
I have been developing a site using an internal web server (192.168.1.4) and I am now testing it on a real IP.
I have changed the $config['] in config and my logon page loads fine.
But when I submit my logon details the session is not populating with my data (username/password/etc).
I know the username/password are correct and everything works if I change base_url back to 192.168.1.4. I have also tried setting cookie_domain to the real IP (it was blank before) but this didn't resolve the problem.
Can anybody shed any light on this?