Connecting IPv6 Routing Domains Over the IPv4 Internet by Brian Carpenter, Keith Moore, and Bob Fink
Everything you wanted to know about the IPv4 to IPv6 transition and more, in this Cisco publication
“Of great concern to transition strategy planners is how to provide connectivity between IPv6-enabled end-user sites (also known as routing domains) when they do not yet have a reasonable (or any) choice of Internet Service Provider (ISP) that provides native IPv6 transport services. One way to provide IPv6 connectivity between end-user sites (when native IPv6 service does not exist) is to use IPv6-over-IPv4 encapsulation (tunneling) between them, similar to the technique currently used in the 6bone [5] IPv6 testbed network.”