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. You 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
This works 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