This paper considers the development of security policy models for networks which can operate in a multilevel security environment. We outline an abstract security policy model which addresses the access control and information flow requirements in a multilevel network. The network security policy is formulated by drawing as much parallel as possible with the computer security policy. The model is defined and the associated security requirements are given and the model is used to prove that the security conditions are not violated by the defined network operations. In developing network security policies, it is necessary to relate them to specific layers of the network architecture. We conclude this paper by outlining another network security policy model with different network subjects and objects supported by different layers in the architecture.