Reserve a DHCP address in AirPort Utility on Mac
You can set up your base station to reserve an IP address for each client computer on your network. When a client computer joins your network, your base station provides the same IP address to that client. This ensures that the IP address of the client computer doesn’t change.
Note: Your base station must be set up to provide IP addresses to client computers on your network using DHCP before you can make a DHCP reservation. See Share an internet connection.
Open the AirPort Utility app on your Mac, located in the Utilities folder in the Applications folder.
In the graphical overview, select the base station you want to configure, then click Edit. You may need to enter a password for the base station.
Click Network, then verify that “DHCP and NAT” or “DHCP only” is selected in the Router Mode pop-up menu.
Click the Add button below the DHCP reservations list, then do the following in the dialog that appears:
Enter a name for the computer in the Description field.
Click the “Reserve Address By” pop-up menu, choose DHCP Client ID or MAC Address, then enter the client ID or the MAC address in the corresponding field.
A DHCP client requesting an IP address can be identified by either the MAC address of the client or a unique DHCP client ID. If you choose to reserve an address by MAC address, enter the MAC address of the device you are connecting in the field. If you choose to reserve an address by DHCP client ID, enter a unique client ID in the field and make sure the device you are connecting is configured with the same DHCP client ID.
Enter the IPv4 address that you would like the base station to reserve for the client.
Click Save to save your changes and close the dialog, then click Update to save the changes.