When you create a closed network with an AirPort Base Station or Time Capsule, the name of the network remains hidden from computers scanning for wireless networks. Users must enter the exact network name to join the AirPort network, and they may be asked to enter a password. Both the name of the network and the password are case-sensitive. A closed network can provide additional security for your network.
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 AirPort in the toolbar, and then click Wireless.
Click Wireless Network Options and then select the “Create a closed network” checkbox.
To join a closed network, choose Other from the AirPort status menu in the menu bar, enter the name of the network, and, if necessary, the password for the network. Check with the network administrator if you don’t know the name or password for the network.