Abstract: This paper proposes a model called the Network Security Reference Model. The Network Security Reference Model enables common concepts concerning network security to be obtained and security issues and technologies to be shared. It can clearly determine points causing network security problems and implementing the countermeasures. And it can also specify relations between specific network security problems and technologies to solve them. Consequently, the Network Security Reference Model makes the systematic consideration of network security possible. This paper describes criteria for classifying most specific security problems of communications networks, and by classifying network security problems according to this criteria, the Network Security Reference Model identifies some contents of Security Subjects in communications networks. The Security Subject describe those security problems in terms of certain common concepts. Furthermore, the Network Security Reference Model describes some contents of Security Functions, Logical Implementation Methods and Security Element Technologies. As communications networks have progressed, it has become more important to guarantee the security of information processing and communication systems that make use of these networks. Consequently, security technologies such as cryptography and authentication have been researched by many researchers. However, little research from a broad perspective has been conducted concerning solutions to security problems that may occur in the whole network itself, or the functions and technologies that the network should accommodate to guarantee the security. Because of this, the Network Security Reference Model had been a necessary scheme to consider issues of network security.