Changed base_url - now sessions dont work

[eluser]Stuart Marsh[/eluser]
More info.
My setup revolves around my firewall forwarding http traffic to my web server.
Original I was accessing the webserver through the LAN so no problems.
Now I am trying to access the webserver with a real-world IP and PHP cookies seem to have a problem with this.
I created a page to set and retrieve a cookie. If I access it using localhost it is fine and data is returned. If I try it through the external IP, nothing is returned.
Is this because the server does not know which cookie to retrieve?