Computer security can be viewed as a set of mechanisms that protect computer systems from unauthorized access, theft, damage and disruption of the services they provide. It includes protection from both internal and external threats. Internal threats can be flaws in a software program or operating system. External threats are unauthorized access or human error. Much of computer security is based on the principle of separation which states that one thing cannot affect another if they are suitably separated. The main mechanisms for achieving separation are physical, temporal, logical and cryptographic. Each of these four basic techniques is in widespread use today and security by separation is one of the fundamental principles of computer security. From an implementation standpoint, however, computer security is usually attained by a suitable set of mechanisms to provide confidentiality, integrity, and consistency. ** network-security-essentials-pdf **