This book attempts to provide a unified overview of the broad field of data and computer communications. The organizations of the book reflects an attempt to break this massive subject into comprehensible parts and to build, piece by piece, a survey of the state of the art. The book emphasizes the basic principles and topics of the fundamental importance concerning the technology and architecture of this field, and provides a detailed discussion of leading edge topics. The book is primarily intended to serve as a textbook in accordance with the syllabus of computer networks offered by various universities in India. The book highlights the basic principles, and contrasts their application in the specific areas of technology. It contains a number of examples, various diagrams and exercises, which enhance its suitability for classroom instructions.