Microsoft Windows [Version 6.0.6001] (C) Copyright 2006 Microsoft Corp. c:\windows>netsh ras ipv6 set negotiation ?
set negotiation [mode = ] ALLOW|DENY
Specifies whether the Remote Access server will allow IPv6 to be
configured for any client connections it accepts.
mode - the mode
ALLOW - permit IPv6 over client connections.
DENY - do not permit IPv6 over client connections.