Controlling how long a computer has a valid IP address

To control how long an IP address is valid for a computer in your AirPort network, set the Dynamic Host Configuration Protocol (DHCP) lease time.

To set the DHCP lease time:

Open AirPort Utility, located in the Utilities folder in the Applications folder on a computer using Mac OS X, and in Start > All Programs > AirPort on a Windows computer.
Select the device you’re setting up, and then click Manual Setup. Enter the password if necessary.
Click Internet in the toolbar, and make “Share a public IP address” or “Distribute a range of IP addresses” is chosen from the Connection Sharing pop-up menu, and then click DHCP.
Enter a number in the DHCP Lease field, and then choose minutes, hours, or days from the pop-up menu.

Using very short leases, DHCP can dynamically reconfigure networks in which there are more computers than there are available IP addresses.

If you have set up DHCP reservations, computers in the list will not get a new DHCP address when the DHCP lease expires.