This book has been developed for data security, network security and Internet security. As such, before we embark on our journey of understanding the various concepts and technical issues to security, it is essential to know what we are trying to protect. What are the various dangers, when we use computers, computer networks and the biggest network of them all, the Internet. It contains a number of features which make the book different from other existing books. This book has been written for the students of B. Tech., M. Tech., MCA, M.Sc., BCA and other diploma courses. Moreover, everything has been explained through self-explanatory figures.