VPN router behind a 1720

[TechNavigator]

I have installed a Cisco 1720 router with a T1-DSU/CSU to interface with our new ISP. It works fine. However, I have a VPN router that I need to give a public IP address to. The problem I am having, is that, I can only set up the 1720 with Private LAN addresses, not public on the inside Ethernet0. So, my question is, how do I setup the 1720 to allow it to pass public addresses to the inside of Ethernet0?? Do I need to setup a DMZ? Do I need to set it up for bridging? - will it do bridging?

Thanks a bunch!!