Many readers have requested that the text be updated to reflect recent changes; many have suggested specific topics and emphasis. Twenty years after its invention, IPv6 is finally gaining acceptance. Voice and video have replaced file transfer as major uses of the Internet. The sixth edition responds to readers’ suggestions by reorganizing and updating existing chapters and introducing new material: Chapters on the early applications of Telnet and FTP have been eliminated to make space for newer material. A new chapter on the Internet of Things considers the use of TCP/IP in a wireless sensor network. A new chapter on Software Defined Networking examines the use of OpenFlow which, although is it not an IETF standard, has become an important part of network and Internet management. The chapter on protocol layering has been moved earlier in the text. Instructors are warned, however, that layering is not a rigid architecture that explains all protocols. Students should see it as a basic but somewhat simplistic guideline that helps us understand protocols. In Chapter 30, for example, we learn that the protocols for a route-over mesh blur the boundaries between layers by adding shims and blending IP forwarding with Layer 2 reachability. Each chapter has been updated to focus on ideas and technologies that are now being used in the Internet. The most significant change consists of integrating the IPv6 discussion with that of IPv4. Each chapter describes a principle, explains the general design, and then proceeds to explain how the principle applies to IPv4 and IPv6. Readers will see that the two versions of IP are closely interrelated and that it is impossible to understand the changes introduced by IPv6 without understanding IPv4.