Even more basic than that, maybe a stupid question, but I know you have been screwing around with your firewall, etc. Have you actually enabled port 80 on the Core's firewall? By default it is disabled...
As long as the firewall is on, you will have to set up a Core_Input rule for port 80 to allow access. Also no port forwarding on the Core on port 80 either, of course or it will just forward the traffic straight through to your internal network!