An IPv4 over IPv6 tunnel connects isolated IPv4 sites over the IPv6 network.
During the later transition phase from IPv4 to IPv6, IPv6 networks have been widely deployed, and IPv4 sites are scattered across IPv6 networks. It is not economical to connect these isolated IPv4 sites with private lines. The common solution is the tunneling technology. With this technology, IPv4 over IPv6 tunnels can be created on IPv6 networks to enable communication between isolated IPv4 sites through IPv6 public networks.
Using IPv6 tunnels as virtual links for IPv4 networks allows carriers to fully utilize existing networks without upgrading internal devices of their backbone networks.