Transition From IPv4 to IPv6

One problem in transition from IPv4 to IPv6 completely is that IPv6 is not backward compatible. This results in a situation where either a site is on IPv6 or it is not. Unlike an implementation of new technology where the newer one is backward compatible so the older system can still work with the newer without any additional changes.
To overcome this short-coming, there exist few technologies which can be used in slow and smooth transition from IPv4 to IPv6:

Dual Stack Routers

A router can be installed with both IPv4 and IPv6 addresses configured on its interfaces pointing to the network of relevant IP scheme.

[Image: Dual Stack Router]

In above diagram, a Server which is having IPv4 as well as IPv6 address configured for it now can speak with all hosts on IPv4 network and IPv6 network with help of Dual Stack Router. Dual Stack Router, can communicate with both networks and provides a medium for hosts to access Server without changing their respective IP version.

Tunneling

In a scenario where different IP versions exist on intermediate path or transit network, tunneling provides a better solution where user’s data can pass through a non-supported IP version.

[Image: Tunneling]

The above diagram depicts how two remote IPv4 networks can communicate via Tunnel, where the transit network was on IPv6. Vice versa is also possible where transit network is on IPv6 and remote sites which intends to communicate, are on IPv4.

NAT Protocol Translation

This is another important method of transition to IPv6 by means of a NAT-PT (Network Address Translation – Protocol Translation) enabled device. With help of NAT-PT device, actual conversion happens between IPv4 and IPv6 packets and vice versa. See the diagram below:

[Image: NAT - Protocol Translation]

A host with IPv4 address sends a request to IPv6 enabled Server on Internet which does not understand IPv4 address. In this scenario, NAT-PT device can help them communicate. When IPv4 host sends a request packet to IPv6 Server, NAT-PT device/router, strips down the IPv4 packet, removes IPv4 header and adds IPv6 header and passes it through the Internet. When a response from IPv6 Server comes for IPv4 host, the router does vice versa.
http://ipv4-ipv6.blogspot.com/2014/07/one-problem-in-transition-from-ipv4-to.html