This book covers the topics related to Physical Layer, Data Link Layer, Network Layer, and Application Layer. The book also explains Router configuration and different LAN Technologies in a very simple language. Separate chapters are added on ATM, ISDN and Frame Relay containing many illustrations. Network Security concepts are explained with easy-to-understand language. Concept of Transmission Control Protocol and User Datagram Protocol are discussed in a matter that the students of various streams can easily understand these complex topics.