CIDR FAQ

> > But isn't it possible to have a router convert from an IPv6 address
> > with provider bits to an IPv4 address by simply stripping all but the

> You can't strip a non-zero part of IPv6 address, as this would cause
> transport layer pseudo-header checksum to fail.

Unless you recalulate the checksum.  There are products out now
that do similar things (like map a large set of private ip addresses to 
a single class C).