Restrict portal application to "Intranet access only"

Hi experts,

I need to deploy my ESS/MSS applications to both internet and intranet, but want to restrict certain applications to only intranet. If employee accesses these intranet application from the public network, they will see a message "restrict to intranet only"

Would like to know how and where i should configure the portal to make this restriction.

Thanks for your replies.

Cheers!

Fred