The names and the IP addresses of client computers are not logged when you enable auditing for a site collection in Windows SharePoint Services 3.0

•	When you view the audit reports, you do not see any information logged about the names or the IP addresses of client computers that accessed the site collection.
•	When you view the AuditData table in the content database of the Web application, the values in the MachineIP column and in the MachineName column appear as NULL.

Reason for that is :

"The values in the MachineIP column and in the MachineName column appear as NULL because of privacy concerns. By design, Windows SharePoint Services 3.0 works in this manner."

Note : I don't know why they giving those feature when it disable.


http://support.microsoft.com/kb/939246