Operate Cisco Router 857w in bridge mode to PIX 506e

I have a Cisco PIX 506e and a Cisco 857w router. I have been using cisco hardware for around a week so i'm still very new to configuring cisco devices.

 The router is configured to connect to the internet ADSL account using PPPoE/CHAP on the router's WAN port. The router's LAN port is connected to the "outside" port on the PIX. The "inside" port on the PIX is connected to the LAN switch.

So the setup is something like this:

[inside LAN workstations] --> [Switch] -->(inside) [PIX] (outside)--> (lan)[Router](wan) --> [Internet]

Currently this functions correctly, the wan port on the router obtains a public IP address and the lan port is on a private IP address and does NAT/DHCP. However the PIX is also running nat/dhcp to it's inside network. THis is okay and everything functions correctly, all of the workstations on the inside network can access the internet.

However I want to operate the router in bridge mode so that it is simply functioning as a dumb modem for the PIX, and the external address will become the outside interface on the pix. How do I configure the router to do this?