Data communication networks have become an integral part of modern society. The standardization of communication protocols has made the interconnection of heterogeneous systems and networks a reality. This global connectivity among open systems provides facilities such as remote computing, resource sharing and electronic fund transfer. For all these applications to provide the services that they are designed to provide in a secure manner, it is necessary to evaluate all the possible security violations that may occur. Moreover, the requirements of applications to protect the transfer of information from a range of potential threats should also be assessed. This paper introduces some of the commonly known security threats, together with the security services and state-of-the-art mechanisms that can be used to provide protection against these threats.