FTA - "Given the global lack of available IPv4 space, and limitations in
IPv4 extension and transition technologies, this document advises
that IPv6 support is no longer considered optional. It also cautions
that there are places in existing IETF documents where the term "IP"
is used in a way that could be misunderstood by implementers as the
term "IP" becomes a generic that can mean IPv4 + IPv6, IPv6-only, or
IPv4-only, depending on context and application."