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