<b>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. ...</b>
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.