Share an internet connection in AirPort Utility on Mac
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, click the Router pop-up menu, then choose “DHCP and NAT.”
Click the Network Options button near the bottom of the pane.
In the dialog that appears, choose a starting range from the IPv4 DHCP Range pop-up menu, then enter the range in the corresponding fields.
Important: During setup, AirPort Utility Setup Assistant automatically determined whether “Enable IGMP Snooping,” “Block incoming IPv6 connections,” “Allow Teredo tunnels,” and “Allow incoming IPSec authentication” should be turned on or off. Don’t modify these settings unless you’re instructed to do so by your ISP or network administrator.
If you set up a guest network, set a range for it by choosing a starting range from the Guest IPv4 DHCP Range pop-up menu, then entering a number in the corresponding field.
Click Save to save your changes and close the dialog, then click Update to save the changes.