Covert router Nat mode to Bridge mode

Hi,
My internet is terminated on xDSL modem from this modem a serial cable goes to my Cisco 1700Router and from router the lan cable goes to my internal network switch. Now i am implementing the Nitix firewall which itself act as a Router but my cisco router is in NAT mode and i need to convert it in to Bridge mode
so that my firewall will work on gateway level security.
For this I need to convert my router from NAT mode to Bridge mode.
Can anyone Please help me out in this issue