IPNetSentryX allows you to configure an AirPort card as a software base station using the AirPort configuration tool. You will need to configure other network software to route packets between your AirPort and Ethernet interface if desired.
Notice the radio used for AirPort can operate in one of three modes:
The AirPort configuration tool allows you to select the desired operating mode, network name (for AP or Computer-to-computer mode), radio channel, encryption mode, and passphrase. Press Apply to apply any changes you have made and add them to the currently open IPNetSentryX document. Saving this document allows you to restore these settings at a later time.
Notice Apple does not currently support WPA (WiFi Protected Access) or WDS (Wireless Distribution System) for software base stations. These might be welcome additions if Apple were persuaded to support them. WPA would allow software base stations to offer security similar to Apple's hardware base stations. WDS is a system for relaying wireless signals using a single receiver and transmitter to extend the range of a wireless network through an Access Point that is not attached to a wired LAN.