This appendix should give a first idea of the concepts of network security. The methodology followed here is to start from a general point of view on security issues, and then to pass on to a more detailed and implementation oriented approach. Complicated implementation details are not discussed here. The section on ISO and Internet protocols is rather technical, but not impossible to read. It is assumed that the reader has some knowledge of the layered approach to networking as proposed in the OSI 7-layer framework. For people with little understanding of encapsulation of packets and the layered approach to networking, the chapter on ISO transport and network layers may be of some help.