Open Port 631

I've set up a CUPS server on CENTOS to act as a gateway and it works fine if I install the printer using:

http://XX.XX.XX.XX:631/printers/printername

But can not get it to work using

https://XX.XX.XX.XX:631/printers/printername

We are on Active Directory 2008 and I was wondering if and how to open port 631, if that helps.

Can't get https to work internally or externally.

thanks