Most Macs have a built-in AirPort card, which provides wireless network access via Wi-Fi (802.11) networks. Assuming the AirPort card is turned on (as it is by default) and all other settings are in their default states, whenever your Mac is disconnected from the Internet and encounters a Wi-Fi network, it asks whether you want to join that network (thereafter prompting you for a password if necessary). And you can freely switch between networks by using the AirPort menu in the main menu bar (the one that looks like a striped wedge) or by using the Network pane of System Preferences.
All this makes it convenient to stay connected to the Internet, and for single-user Macs (or portable Macs), it's most likely what you want. However, in some settings — for example, a school or office where Internet access must be tightly regulated — you may not want users to be able to freely switch between wireless networks. In these cases, you can lock down certain AirPort settings so that they can be changed only when an administrator's password is typed.
To configure your AirPort security settings, follow these steps:
Choose System Preferences to open System Preferences and then click Network to open the Network pane.
If the lock icon in the lower-left corner of the pane ...