You can create tunnels on the IPv6 networks to connect IPv4 isolated sites so that IPv4 isolated sites can access other IPv4 networks through the IPv6 Internet.
During the transition from the IPv4 Internet to the IPv6 Internet, IPv6 networks have been widely deployed, whereas IPv4 networks are isolated. The tunnel technology can be adopted to establish tunnels over IPv6 networks to connect isolated IPv4 networks. This is similar to the situation where the tunnel technology is used to deploy VPNs on the IP networks. The tunnel used to connect isolated IPv4 networks over IPv6 networks is called an IPv4 over IPv6 tunnel.