Windows to mac IP restriction problem

[mikej]

We have a Windows Pro 2000 desktop machine serving out FMPro 5 unlimited in a Uni department. We have small booking databases for car bookings and IT help requests, accessed by a Departmental web page. Since IP restriction was turned on (Windows server) to give some level of security, Macintosh computers can see, but no longer submit bookings, if using Internet Explorer, Safari or any non Netscape/Mozilla based browser. It does not matter if the mac OS is classic or OSX. Netscape or Mozilla work, IE and Safari do not. Does anyone have any idea where the problem may lie?