Switched Ports
From DD-WRT Wiki
With an older router, you can make for instance ports 3 and 4 to reside outside the router's NAT. Devices attached to these ports will request an own IP address from the ISP instead of getting a private IP address from your WRT. One might want to have such behaviour if you're having unsorted firewall issues with your SIP operator.
nvram set vlan0ports="1 2 5*" nvram set vlan1ports="0 3 4 5" nvram commit reboot
According to user reports, this is confirmed to work at least with a WRT54GSv1.1. If you want to recover the default settings, write the following within ssh session:
nvram set vlan0ports="1 2 3 4 5*" nvram set vlan1ports="0 5" nvram commit reboot