Data Communications Networking Devices - 4th Ed.pdf

2.5 THE INTERNET _______________________________________________________________ 233Figure 2.39Link-local and site-local addresses facilitate connecting private networksThe lower portion of Figure 2.39 illustrates the format of a site-local addressunder IPv6. This address would be assigned to a site that has routers but is notcurrently connected to the Internet via an ISP. When it's time to connect the siteto the Internet, the router would be con®gured with a new pre®x that would ineffect generate a provider-based address through the inclusion of Registry ID,Provider ID, and Subscriber ID ®elds. Since no portion of the site-assigned partof the address would require changing, the connection of the site to the Internet ismore gracefully accomplished under IPv6 than under IPv4.Now that we have an appreciation for a few of the new types of addressessupported under IPv6, let's turn our attention to migration issues associated withmoving to IPv6.Migration issuesRecognizing reality, there will be no magic data for the `mother of all cutovers'from IPv4 to IPv6. Instead, you can expect a gradual and incremental upgradeprocess to occur an you obtain new IPv6-compliant equipment for use. However,there is no cause for alarm for organizations that are happy with their current blockof IP addresses and prefer the status quo. Such users will probably be able tocontinue using IPv4 for many years or until their existing equipment is graduallyreplaced by IPv6-compliant products and they elect to migrate. Thus, let's turnour attention to the two basic methods or migrating to IPv6 and how the migrationprocess provides backward compatibility with IPv4.Migration methodsThere are two methods you can consider for migrating to IPv6Ðdual stacks andtunneling. Under the dual stack method each IP node becomes capable ofsupporting both IPv4 and IPv6 and is referred to as an IPv6/IPv4 node. Since theIPv6/IPv4 node can transmit and receive both IPv4 and IPv6 packets, it becomescapable of operating with both IPv4 and IPv6 nodes. Figure 2.40 illustrates a nodeoperating a dual IPv6 and IPv4 stack. Here the IPv6/IPv4 node would becon®gured with a 32-bit address per interface to support IPv4 functions and 128-bit address per interface to support IPv6 functions.