About the Guest network feature of AirPort Extreme and Time Capsule

A Guest network is a second, separate network provided by your AirPort Extreme or Time Capsule. It allows guest network clients to access the Internet (and use Internet-related applications such as iChat and Safari).

Notes

• You can secure the Guest network with its own password (WPA or WPA2) if you wish.
• When setting up a Guest network, it is a good idea to use a name that easily distinguishes it from your primary network. By default, AirPort Utility will append the word "Guest" to your Guest network name. 
• If you would like to allow your Guest network clients to communicate with each other via File Sharing, open AirPort Utility and enable the "Allow guest network clients to communicate with each other" option under the Guest Network tab.
• In order to use a Guest network, your AirPort Extreme or Time Capsule Wi-Fi base station must be sharing a public IP address via DHCP for client computers. While configured in this way, your AirPort base station or Time Capsule acts as a router, taking the IP address provided by your ISP and distributing multiple, private IP addresses for clients of the AirPort network. This is the default behavior of the AirPort base station or Time Capsule and can be changed using AirPort Utility. 
• If you have multiple AirPort base stations, your AirPort Extreme or Time Capsule should be configured as the primary base station in the wireless network in order for you to enable the Guest network feature.
• If you want to give guests access to network features besides the Internet (such as AirPort printers, Time Capsule disk(s), AirTunes speakers, or AirPort disks associated with your primary network), consider using the Add Wireless Client feature. This will allow clients to join the primary AirPort network either for a day or longer without requiring the password. 
• Access Control lists will apply to both the primary network and the Guest network. If you use Access Control lists, you will need to add your Guest network clients to the list in order for them to join your Guest network.