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For more information on this, take a look at this article:
When do Apple wireless devices appear in AirPort Utility?
Troubleshooting missing devices
If your Apple wireless device doesn't show up in the wireless device list, try the following:
- Check to see that the device is turned on.
- Check that the device is connected to the network via Ethernet or in wireless range of your computer.
- Make sure the device has had adequate time to start up and become discoverable. In the case of Apple wireless devices that are not connected via Ethernet, AirPort Utility scans for new devices once every 15 seconds. From the time a device becomes discoverable (typically around one minute), it could take up to another 15 seconds before the device is displayed in the list.
- If you have just configured a new Apple Wireless device, upgraded the firmware, selected "Restore Default Settings..." or have hard reset and configured an Apple wireless device and it does not show up in AirPort Utility after restarting, you may have an incorrect setting. You may want to hard reset the unit and set it up again.
- If you are connected to a specific wireless network, all Apple wireless devices which make up that network will appear. However, if you "Restore Default Settings..." from AirPort Utility, or hard reset a device that is out of physical wireless range of the computer, you will need to move the device closer the computer to configure it again.
Best of luck,