From DD-WRT Wiki
The orig firmware is based on openwrt.
 Initial flashing instruction
For initial flashing just flash the appropriate DD-WRT overtake image file (WW (Worldwide), NA (North America)) though the web interface. After the reboot DD-WRT will answer at 192.168.1.1. Future upgrades are done though the GUI to the latest builds.
As of 20140119 you can only install DD-WRT if your Netgear fw version is < 18.104.22.168 (ticket). Netgear decided to limit the orif fw size, so you will have to downgrade orig firmware to be able to flash to DD-WRT.
If you have a bricked device or you want to go back to the original firmware you can flash the device useing the recovery boot procedere.
- press and hold the reset button
- powercyle the device
- keep holding the button until the power led flashes
- start to transfer the firmware with an tftp client to 192.168.1.1
- when the transfer begins release the reset button
- wait until the unit reboots