AirPort Express can‘t join network in client mode, flashes its status light (LED)

If you want to use your AirPort Express in client mode but unwittingly try adding it to a network that is protected with an access control list (ACL), AirPort Express will flash its status light amber once per second. This means that it wasn't able to join the network and get an IP address.
The ACL is stored on the base station that is hosting the wireless network, and it contains the MAC (media access control) address of each device that is allowed to join the network. To use your AirPort Express as a wireless client, its MAC address must be added to the ACL.

AirPort Express has two MAC addresses, and both are listed on its case. The one labeled "AirPort ID" is the MAC address for its wireless interface, and the one labeled "Ethernet ID" is the one for its Ethernet port.

If you are the network administrator, you simply need to update your ACL using AirPort Admin Utility so that it includes the "AirPort ID" of the AirPort Express. If not, speak with the person who runs the network.
Published Date: Oct 10, 2016