This book is now in its sixth edition. Each edition has corresponded to a 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. By the fifth edition, networks were about content distribution (especially videos using CDNs and peer-to-peer networks) and mobile phones. Now in the sixth edition, industry emphasis on is very high performance, with 5G cellular networks, 100-gigabit Ethernet, and 802.11ax WiFi at speeds up to 11 Gbps just around the corner.