With AirPort software version 2.0.4 you can set up a DHCP messaging feature that sends a message, such as network information or a greeting, to connecting DHCP clients (see Notes 1 and 2).
How to Set Up DHCP messaging
Follow these steps:
1. In Mac OS X, open AirPort Admin Utility version 2.0.4.
2. Click the Network tab.
3. Enter the message or greeting in the Message field, which appears in the lower right corner (Figure 1).

Figure 1 The Network pane of AirPort Admin Utility
4. Click Update (see Note 3).
Once the base station has been updated, any Mac OS 9 client connecting to the base station will see the message when it receives its IP address via DHCP from the base station (Figure 2).
Figure 2 A DHCP message in Mac OS 9
Notes
1. Only Mac OS 9 clients will see DHCP messages.
2. Only the AirPort Base Station (Dual Ethernet) can be configured to use this feature.
3. The message must be 253 characters or less. If you click Update with more than 253 characters, an alert box with this message appears:
"The DHCP message cannot be more than 253 characters. Please remove <number> or more characters and try again."