One of the conceptually easiest ways of introducing IPv6 to a network is called the “dual stack mechanism”, as described in [NG05], which is an update of RFC 2893 [RFC2893]. Using this method a host or a router is equipped with both IPv4 and IPv6 protocol stacks in the operating system (though this may typically be implemented in a hybrid way). Each such node, called an “IPv4/IPv6 node”, is configured with both IPv4 and IPv6 addresses. It can therefore both send and receive datagrams belonging to both protocols and thus communicate with every node in the IPv4 and IPv6 network. This is the simplest and most desirable way for IPv4 and IPv6 to coexist and is most likely to be the next step in a network’s evolution in general, before a wider transition to an IPv6-only Internet can be achieved worldwide (in the long term future).

There are no real transition mechanisms to use within the dual stack scenario, as “Dual Stack” is a method to integrate IPv6 itself.

One challenge in deploying an IPv6/IPv4 Dual Stack network lies in configuring both internal and external routing for both protocols. If one has for example used OSPFv2 for intra site routing before adding IPv6 to the Layer 3 network one will either have to transition to a protocol, which is both IPv4 and IPv6-capable like IS-IS or be forced run one IS-IS or OSPFv3 in addition to OSPFv2. Since configuring IPv6 routing in a dual stack network is usually completely independent from the configuration of IPv4 routing the reader can refer to Chapter 6 for most of the issues concerning basic routing setup.

Another challenge lies in the interaction of the two protocols, and how this interaction is managed, given that a dual-stack network will (in an early stage of worldwide IPv6 deployment) generally be interacting with IPv4 external networks. An example is the deployment of email servers for SMTP, and how the MX servers are provisioned for both protocols by offering IPv4 or IPv6 reachability, and how failover is handled between the protocols.