This book is the official supplemental textbook for the Network Fundamentals course in the Cisco® Networking Academy® CCNA® Exploration curriculum version 4. The course, the first of four in the new curriculum, is based on a top-down approach to networking. The Companion Guide, written and edited by Networking Academy instructors, is designed as a portable desk reference to use anytime, anywhere. The book’s features reinforce the material in the course to help you focus on important concepts and organize your study time for exams.