In implementations of DHCP before Windows 2000, any user can make a DHCP server on network, an action that can rise conflicts in IP address allocation. In Windows Server 2000 and 2003, an unauthorized DHCP server is a DHCP server that has not been openly listed in Active Directory service as an authorized server. You must authorize a DHCP server in Active Directory before server can issue rent to DHCP clients.

· Open DHCP console under Administrative Tools.
· In the Console Tree, right click DHCP>> select Manage Authorized Servers.
· Under Manage Authorized Servers box, click Authorize.
· In Authorize DHCP Server box, enter name or IP address of DHCP server to be authorized and press OK button.
· The PC will list IP and PC name and then ask for validation. Press OK button.