The management of large data networks, like a national WAN, is without any doubt a complex task. Taking into account the constantly increasing size and complexity of today's TCP/IP based networks, it becomes obvious that there is a demanding need for better than simple monitoring management tools. Expert system technology seems to be a very promising approach for the development of such tools. This paper describes the system architecture of ExperNet, a distributed expert system for the management of the National Computer Network of Ukraine, and the implementation of the tools used for its development. ExperNet is a multiagent system built in DEVICE, an active OODB enhanced with high level rules, that uses CS-Prolog II to implement the communication facilities required. The system employs HNMS+ and Big-Brother, two modified versions of existing network management tools, in order to obtain a complete view of the monitored network.