To cope with the complexity and broad scope of areas and functions that are involved with a network management system (NMS), a knowledge model should be provided for the system. The aim of this paper is to propose the design of a multi-agent based network management support system with well-organized representation of the formal semantics of networks' experiential knowledge related to NMS fault diagnosis functional area. The modeling of the network knowledge is performed using domain ontology, fault-state causal reasoning models, and problem solving methods (PSMs) in a generic and reusable manner. Hence, knowledge acquisition in the NMS domain can be done by using these reusable structures, thereby eliminating the need to build the new knowledge-based systems from scratch. This paper presents an approach towards the agent-mediated ontology-driven automated NMS to benefit the network administrators through the automatic provision of just-in-time and context-dependent knowledge for resolving network failures.