RFC 1122, section 3.2.2.9:                                  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:

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Tcpip\Parameters

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.