Is it true that the OS X firewall needs to be turned off when you are using the AirPort Express ... AND .... do any of the advanced settings (Stealth, UDP blocking) have any affect if it is indeed turned off?
One more thing .. if the firewall is turned off, do opening/blocking any specific ports make any difference?
One more thing .. if the firewall is turned off, do opening/blocking any specific ports make any difference?