Network Load Balancing hosts that operate in unicast mode have never been able to communicate with each other because NLB makes the MAC ( Media Access Control) address the same on all systems.

After applying Service Pack 1, or later, to Windows Server 2003, you can enable unicast inter-host communications support by running UnicastInterHostCommSupport.bat.

The syntax for running UnicastInterHostCommSupport.bat is:

UnicastInterHostCommSupport ClusterIPAddress

Where ClusterIPAddress is the IP address of the cluster.

UnicastInterHostCommSupport.bat contains:

@echo off                              if \{%1\}==\{\} @echo Syntax: UnicastInterHostCommSupport ClusterIPAddress&goto :EOF                              setlocal ENABLEDELAYEDEXPANSION                              set IP=%1                              set IP="%IP:"=%"                              set qry=reg query HKLM\System\CurrentControlSet\Services\WLBS\Parameters\Interface                              for /f "Tokens=*" %%g in ('%qry%^|findstr /B /L /C:"HKEY_LOCAL_MACHINE"^|find "\{"^|find "\}"') do (                               for /f "Tokens=2*" %%h in ('reg query %%g /V ClusterIPAddress^|find %IP%') do (                                @echo reg add %%g /V UnicastInterHostCommSupport  /T REG_DWORD /F /D 1                                reg add %%g /V UnicastInterHostCommSupport  /T REG_DWORD /F /D 1                                NLB RELOAD                               )                              )                              endlocal