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 dhcp addresses to devices that connect to it. It is a 'dhcp server'. A dd-wrt router in default configuration is also a 'dhcp server' However, a LAN network can only have ONE dhcp server. You need to shut off one of these DHCP servers, or create a subnetwork.
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