Internet Sharing Problems from iMac

Hey Guys,

I'm at university and have been connected using ethernet with no problems for the past two weeks. I have also been sharing the internet connection from my iMac to my Macbook Pro, iPad and iPhone with little problems.

The university network operates on a "Keep Alive" method, so I have to keep a tab opened to keep the internet connected after logging in with my university name and password. If this tab is closed, I don't have internet.

However, I woke up this morning and after connecting the normal way, my Macbook, iPhone or iPad can connect to the shared network, but they cannot to the internet. My MacBook Pro is displaying the "Alert: No Internet Connection".

Any solutions? I have restarted all devices, and set up a new shared WiFi network, but nothing has worked.

Internet works fine on the iMac I am connected to the ethernet with, it's just on the shared computers, which can connect to the network, but not to the internet.