Lock down a device address in AirPort Utility?
Try the steps detailed at this URL: http://www.macinstruct.com/node/553


Find your Mac's MAC address for the Wi-Fi network interface. See How to Find Your Mac's MAC Address for instructions.
Open the AirPort Utility application. (It's in Applications → Utilities.) The window shown below appears.
Click the AirPort Extreme's icon. The status pop-up window appears.
Click Edit. The settings window appears.
Select the Network tab. The window shown below appears.
Click the + button under the DHCP Reservations field. The window shown below appears.
Enter a descriptive name for your Mac in the Description field.
From the Reserve Address By menu, select MAC Address.
Enter your Mac's MAC address in the MAC Address field.
Enter the static IP address you want to assign to your computer. For more information, see step 9 in How to Set a Static IP Address in Mac OS X.
Click Save.
Click Update. The AirPort will restart to apply the changes.