In Windows 2000 and Windows Server 2003, Unicast NLB nodes cannot communicate over the NLB-enabled network adaptor. In order to enable the communication between the Unicast NLB nodes, we have to install additional network cards on the computers so that they communicate using the additional network cards.
In Windows Server 2003, Microsoft has implemented the feature for Unicast NLB nodes communicating over the NLB-enabled adapters. In order to enable this feature, the registry value UnicastInterHostCommSupport needs to be added.
Please refer to this Knowledge Base article for more information.
898867 Unicast NLB nodes cannot communicate over an NLB-enabled network adaptor in Windows Server 2003