IPv4 has been a huge success that goes beyond the dreams of its inventors. However, the IPv4 addressing space is far too small to cope with all the needs for Internet connected hosts. IPv6 is slowly replacing IPv4 and deployment continues. The plot below shows the growth in the number of IPv6 browsers worldwide.

- source : Eric Vyncke’s Projection of IPv6 %-age of IPv6-Enabled Web Browsers (courtesy Google) in World Wide

As of today, most IPv6 enabled devices, except in some large mobile networks, are dual-stack hosts that have both IPv4 and IPv6 addresses. Mixing IPv4 and IPv6 helps in deploying IPv6 gradually, but also consummes IPv4 addresses. In the future, more and more hosts will be only attached to IPv6 networks and they will use specific transition mechanisms such as NAT64 or 464XLAT.

In an IPv6-only network, hosts that also support IPv4 may try to use this address family and create link-local addresses. This could lead to operational issues. A recent Internet draft, draft-hinden-ipv4flag proposes a new flag to the ICMPv6 Router Advertisements to announce the unavailability of IPv4 in an IPv6-only network. A good move towards deprecating IPv4 one day…