At the highest level of description, this book is Introduction to Computer Networks. It focuses on Basic level of networks and its background of networks. This book is not intended as an introduction to Computer Networks, although we do provide the background necessary in several areas in order to facilitate the reader's comprehension of their respective roles in Networking. This book reviews state-of-the-art. This is the first book that explains how computer networks work inside, from the hardware technology up to and including the most popular Internet application protocols.