RFC 1122, section 188.8.131.52: 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. ...Prior to SP4, Windows NT responded to Address Mask requests.
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.