I second the reality view of Dan Bernstein, and:
In the message <20011112210939(_dot_)1915(_dot_)qmail(_at_)cr(_dot_)yp(_dot_)to>
at Mon, Nov 12, 2001 at 09:09:17PM -0000,
"D. J. Bernstein" <djb(_at_)cr(_dot_)yp(_dot_)to> writes:
Under the RFC 2893 transition plan, the existing IPv4 address space is
included in the IPv6 address space. If this is done properly, it can
entirely eliminate the administrative hassle.
And from the programming point of view, using IPv4-mapped IPv6 address
space (::ffff:/96, RFC2373 Section 2.5.4) can eliminate the issues on
running an IPv6 program on an IPv4-only node. djbdns with this
modification runs happily on an IPv4-only environment, though it treats
IP addresses as uniformed 128bit integers internally.
As Dan writes, the IPv4 address space should be fully included in the
IPv6 address space.
// Kenji Rikitake