When I signed up for internet with my local ISP, they ended up supplying me with a modem router combo that left a lot to be desired for the router portion. I had an Apple airport extreme router that I wanted to use, but in order to interface the airport router with the router supplied by my ISP I had to disable NAT and DHCP in that router to essentially turn it into a passthrough. Below are the steps I used to put my ISP router in "Bridge Mode". these steps are specifically for the Motorola SBG6580, but these step can generally be applied to any router.
Step 1: Disconnect Coaxial Cable from SBG6580
By disconnecting the coaxial cable it allows us to make the changes required to put the modem in bridge mode.
Step 2: Reset The SBG6580 to the Default Settings
The guarantees that the router has all of the factory default settings. To reset the modem hold down the reset button on the back of the router using a pin.
Step 3: Log Into Router
Hook up an ethernet cable from your PC or Mac to the WAN port 1 on the router. log into the router by typing 192.168.0.1 into your browser. The login screen for the router should appear. Insert the default username and password into the login screen. For the SBG6580 the default username is "admin" and the password is "motorola".
Step 4: Disable Primary Wireless Connection
In order to disable the wireless connection go to Wireless > Primary Network and set the "Primary Network" item to "Disabled" click apply at the bottom of the screen.
Step 5: Copy Router WAN MAC Address and Add as Passthrough
Go to Basic > Configuration and copy down the WAN MAC address for your router listed there. Go to Advanced > Options and add the MAC address copied down to the "Passthrough MAC Address" section at the bottom of the page.
Step 6: Disable DHCP Mode
Go to Basic > DHCP and set the "DHCP Server" to "No" and click "Apply" to save the changes. Make sure that your computer is connected to the WAN port 1 otherwise you will not be able to re-connect to the router. Log back into the router after it has completed rebooting.
Step 7: Disable NATP Mode
Go to Basic > Setup and set the "NATP Mode" section to "Disabled" and click "Apply" to save the settings. Once the router has rebooted in order to log into the router you will have to enter 192.168.100.1 instead of 192.168.0.1 to enter the configuration menu for the router.
Step 8: Reconnect Coaxial Input and Second Router
Unplug your computer from the WAN port 1 on the router and connect the second router to WAN port 1. Reconnect the coaxial cable and power cycle the routers. The first router should now pass through the IP address and other settings to the second router. Hook up your computer to the second router and verify that you can connect to the internet.