Modem - Connection to Router
From DD-WRT Wiki
To connect your router to a modem, it is important to note that a modem is designed to provide you with a connection to the WAN. The modem is generally the link between your LAN and the ISP.
The ISP could assign you a Static WAN IP or a Dynamic (DHCP) WAN IP address and configuring of your main router is dependent on the type of IP your ISP provides.
For your LAN you will probably want a DHCP server and this is where your main Access Point Router comes into play. This is the host unit that supplies LAN addresses to the rest of your network and should be the only DHCP server on your base IP subnet. Remember there should only be one DHCP server per subnet
If your modem does not have a router built in, you should be able to plug the modem cable into the wan port and have the router get a WAN IP address from the modem in the top right corner. If you do not get a WAN IP, turn off the modem for ten minutes, and unplug the router. Then plug in the modem, and then plug in the router. If you don't get a WAN IP, you might have to clone your former unit's mac address. Make sure the mac address for your router in the webgui matches the one on the label. Further information on this is at note 14 of the peacock announcement: http://www.dd-wrt.com/phpBB2/viewtopic.php?t=51486
If your modem has a router built in, you would want to shut down the DHCP server on either the modem, or on your host router.
To shut of the dhcp server in the modem, you need to set the modem into a 'bridge mode'. This would be done following the instructions from your ISP, as each modem would be different.
To shut off the dhcp server in your dd-wrt, disable dhcp server in the basic settings page.
 DSL Modems
DSL Modem connection info needed.
 Cable Modems
Cable Modem info needed