Microsoft Knowledge Base Article 890161 contains the following introduction:

This article describes the effects of removing null sessions from the Microsoft Windows 2000 and Microsoft Windows NT environments. Specifically, this article discusses the following topics:

• Enumeration in the Windows 2000 and Windows NT environments
• Null sessions and enumeration
• Using the RestrictAnonymous registry value to control null sessions
• The effects of removing null sessions from the Windows 2000 and Windows NT environments