different phase in the way computer networks were used. When the first edition appeared in 1980, networks were an academic curiosity. When the second edition appeared in 1988, networks were used by universities and large businesses. When the third edition appeared in 1996, computer networks, especially the Internet, had become a daily reality for millions of people. By the fourth edition, in 2003, wireless networks and mobile computers had become commonplace for accessing the Web and the Internet. Now, in the fifth edition, networks are about content distribution (especially videos using CDNs and peer-to-peer networks) and mobile phones are small computers on the Internet. New in the Fifth Edition Among the many changes in this book, the most important one is the addition of Prof. David J. Wetherall as a co-author. David brings a rich background in networking, having cut his teeth designing metropolitan-area networks more than 20 years ago. He has worked with the Internet and wireless networks ever since and is a professor at the University of Washington, where he has been teaching and doing research on computer networks and related topics for the past decade. Of course, the book also has many changes to keep up with the: ever-changing world of computer networks. Among these are revised and new material on Wireless networks (802.12 and 802.16) The 3G networks used by smart phones RFID and sensor networks Content distribution using CDNs Peer-to-peer networks Real-time media (from stored, streaming, and live sources) Internet telephony (voice over IP) Delay-tolerant networks