Transitioning to IPv6

There will be no flag day - we will transition gradually

"Dual stack"
Every IPv6 nodes in the early stage will be capable of speaking IPv4
IPv4/v6 nodes will use IPv6 to communicate with IPv6 nodes
IPv4/v6 nodes will use IPv4 to communicate with IPv4 nodes
The use of IPv6 will gradually increase, and become dominant

"Tunnelling"
IPv6 networks may be at distant locations, not directly adjacent to each other
Encapsulate IPv6 packet into IPv4 to deliver packets through IPv4 networks
Just like mbone/VPN tunnels