A system MUST NOT send an Address Mask Reply unless it is an
authoritative agent for address masks. An authoritative agent
may be a host or a gateway, but it MUST be explicitly configured
as an address mask agent. ...</b>
Upgrade to SP4.
If you wish your computer to be an Address Mask agent, navigate to:
Add Value name EnableAddrMaskReply as a type REG_DWORD. A value of 0 is the default and prevents responses to Address Mask requests. A value of 1 allows responses.