his continuing phase of Internet growth. The next phase of Internet growth will be different and much larger. Instead of con- necting people, it will to be connecting 'things'. The current phase of Internet growth is making the Internet broader and taller - the next phase will make it denser. We are moving toward a world where more and more 'things' are connected, devices that are not directly associated with people, for example, sensors, appliances, entertainment equip- ment, lightning controls, power distribution, and cars. Just about everything will have a computer in it and will be connected to the Internet. For this phase of Internet growth, IPv6 is essential. Overall, IPv6 solves the problem of addressing in a much, much larger Internet. The Internet has changed the world in many ways; IPv6 will allow the Internet to continue growing and with that growth continue the benefits that it brings to the world. IPv6 running on cellular networks will have an important role in the continuing growth of the Internet. I worked at Nokia from 1998 through 2009 in various groups and roles, and ended with the title of Nokia Fellow. I got to know Jouni, Teemu, and Jonne very well. I was honored to be asked to write the foreword to their book. They were very involved in the 3rd Generation Partnership Program (3GPP) standardization effort and were responsible for bringing IPv6 to 3GPP and making it part of the mobile protocol standards. I believe that this book will make an important contribution to IPv6 deployment in mobile Internet devices. In the IETF there is a toast we like to give. To paraphrase: 'Kudos to Jouni, Teemu, and Jonne for writing this useful book and to the universal deployment of IPv6. Robert Hinden, Palo Alto, California Preface The story of this book began in March 2010, when John Wiley and Sons Ltd approached Teemu with a request to review a book proposal. While reviewing the proposal, Teemu got an idea for a book, and thought it would be great fun to write such a book. And after all, it could not be tha