 changed from multihoming into describing the basics of IPv6 in 3GPP networks, as we thought that there is more need to describe how IPv6 is implemented in 3GPP cellular systems, than focusing on advanced uses of the IPv6. The 400 pages was roughly the size of the book proposal that Wiley approved in February 2012. At the completion, the book had 398 pages. We agreed on a schedule we thought would be doable and would bring the book to readers around mid 2013. Based on the experience elsewhere, nine months should be Preface enough to complete a project anyway. The bold intent was to work on the book in our spare time, almost as a hobby, alongside our daily works duties. Perhaps influenced by the design of TCP, we got into slow start mode. We lived at different cities, and hence our working mode was effectively based on telephone, email, and an IPv6-only SVN-based version control tool that Jouni set up for us. Our face-to-face meetings were limited to IETF meetings, which we all participated in. At the midpoint of the project we had less than a third of the book written. This resulted in quite a busy August-October of 2012, when the bulk of the book was written and reviewed. We did not want to slip on the mid 2013 target, SO we had to steal time from elsewhere - typically from sleeping hours. Despite the long hours towards the completion of the book, we have found this topic fun and educative to write about. The IPv6 is a fascinating technology with many details and aspects. It provides topics of pure academic interest, engineering beauty, fixes and patches, politics and economics, research opportunities for the future, and overall it reflects how human beings work and build the world. From simple details we build very complex systems, which borderline being fully understandable for a single mind. We hope that this book helps you to gain foot hold on IPv6 itself, and in particular in 3GPP systems. Reading this book should provide you with an knowledge framework of this technology, and thus help in applying the know