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Marina Gil-Santamaria, director at Ipswitch Network Management Division looks at strategies for migrating to IPv6
 For years, the Internet protocol known as IPv4 was sufficient for existing demand, but over the past two decades, it became clear the number of possible IPv4 addresses would eventually fail to meet demand. Realising this, engineers and network designers established a new version of the IP protocol called IPv6 with a vastly larger number of potential addresses. But how should organisations move from IPv4 to IPv6 without disrupting their operations?
Very few organisations are in a position to make a wholesale change from IPv4 to IPv6 overnight. Making a ‘big bang’ adoption of IPv6, in which there is a wholesale move from one platform to another, is fraught with risks. You need to be sure that you have assessed all applications and how they will function with IPv6, planned for and updated every piece of network infrastructure and tested every client device that it is IPv6 compliant. As a result, most of us will be living in a hybrid networking world while we make the transition from IPv4 to IPv6.
There are three strategies for migrating to IPv6 in a phased approach.
Dual-Stack Approach to Migrating from IPv4 to IPv6
Networking services are implemented by a number of software types. Some software works at low levels close to the hardware while others provide high-level services to applications. Each layer provides services to the layer above it. The link layer works closest to the network and provides services to the Internet layer, which is where the IP protocols operate. These layers interact, making calls for services to the layer below and providing services for the layer above; thus, it is essential that they are coordinated. You cannot, for example, change the IP protocol in the Internet layer without affecting the transport layer which would, in turn, affect the application layer. One way to deal with this reality when migrating to IPv6 is to implement two stacks: one for IPv4 and one for IPv6.
The advantage of a dual-stack approach is that it provides for flexible operations. Devices supporting both stacks can function with other devices supporting either (or both) of the IP protocols under discussion. A drawback of the dual-stack approach is that running two stacks consumes more memory and there may be additional computational overhead as well.
Translation Between IPv6 and IPv4
If we think of IP protocols as languages, it seems that translation is one obvious method for dealing with the differences in protocols. Designers of IPv6 planned for a transition period in which networks would have to support both IPv4 and IPv6. The IPv6 protocol includes support for translating packet headers from the IPv4 format to the IPv6 format. This support is accomplished by mapping IPv4 addresses to a special subset of IPv6 addresses known as IPv4-translated addresses. The primary advantage of translation is that it avoids the overhead of the dual-stack approach. There are, however, disadvantages. Translation is not always a viable option when network address translation (NAT) is used with IPv4.
The tunneling approach
This method is sometimes used to support protocols that are not directly supported on a network. With protocol tunneling, information about the IPv6 packet’s destination address is encapsulated in an IPv4 packet, so there needs to be a method for determining the address the IPv4 packet should be routed to. Network administrators have a couple of options, either configured tunneling, where the encapsulating packet contains information about the destination address of the IPv6 packet, or automatic tunnelling where the final destination address is determined using an IPv4-compatible address of the IPv6 packet, which is the IPv4 address prefixed with 96 bits of 0s.
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