A irPort 4.2 is a software update that corrects two issues related to the networking protocol.
The new version "fixes compatibility with Wi-Fi Protected Access (WPA2) security software on AirPort Express and AirPort Extreme base stations. This update also resolves IPv6 link local configuration issues."
The update is available for Mac OS X 10.3.3 and later and Mac OS X 10.4.2. A Windows update, for managing AirPort Express and AirPort Extreme base stations, also "resolves issues with WPA2 on Windows XP with Service Pack 2 and Microsoft update KB893357."