We are using a textbook. You must purchase it.
Textbook: Computer Networking: A Top-Down Approach Featuring the Internet, James Kurose & Keith Ross, Pearson. There are 6th edition (2013) and 7th edition (2017) versions of the book. You may purchase or rent either one.
Electronic copy: The 7th edition is available as an electronic textbook at the VitalSource webpage for the book. This will allow you to read the book while online. If you want to read it offline, or if you find online access is too slow, then download a VitalSource Bookshelf app that supports offline access. I recommend that you print the pages with assigned problems.
Paper copy: The 7th edition is available at the UCI bookstore, at the Pearson webpage for the book, and at many other bookstores. You can also find used copies of the 6th edition. Don't buy the 1st through 5th editions, as they don't have all of the material and problems.