Hey razorblack,
You might want to check out these links:
https://technet.microsoft.com/en-us/library/cc707697.aspx
http://blogs.msdn.com/b/ameltzer/archive/2008/02/14/firewalls-and-internet-based-client-management-part-2-isa-bridging.aspx
http://blogs.technet.com/b/jchalfant/archive/2015/04/15/prerequisites-for-ibcm-in-configuration-manager.aspx
Good luck and let me know if you get it working!