Managing a large data network, such as a national WAN, is a complex and cumbersome task. Existing network management software cannot meet the requirements of the increasing size and complexity of today's TCP/IP-based networks, because, in most cases, it provides only simple monitoring tools. We can't fully exploit a WAN without user-friendly, intelligent, network management software enhanced with diagnostic and decision support services.Expert systems provide a feasible and elegant solution in this direction. Currently, the development of expert systems for WAN management is only at the research and experimental stages. Formalizing such a task is difficult because network state information is usually inadequate and incomplete, the scale of behavior characteristics is large, and the network environment continually evolves. Moreover, an efficient network-monitoring schema must exist. 1 The implementation of such functionality must be in accordance with existing network monitoring technology-namely, the Management Information Base (MIB) and the Simple Network Management Protocol (SNMP)-to control existing real-world network devices.Network management also requires coordination among operators of autonomous network nodes. To closely resemble network management's distributed nature, multiagent problem-solving techniques can help model the functionality of individual agents as well as the interactions taking place between them. 2 Multiagent technology is preferable to other distributed problem-solving techniques because it offers enhanced modularity, reactions to environmental changes, and reusability. This article presents the architecture, implementation, operation, and evaluation of the ExperNet system, a multiagent expert system that we developed for Ukraine's national TCP/IP WAN under the framework of a joint EUfunded project. ExperNet helps network operators manage a WAN with its• monitoring tools for capturing network state; • platform for logic-based applications; • efficient and extensible expert system shell; • fast heuristic detection, diagnosis, and repair of network failures; • cooperative problem-solving strategies; and • user-friendly Web-based interface.The system architecture ExperNet consists of a hierarchically structured architecture, with each level consisting of one or more management nodes (see Figure 1). Each node encapsulates one or more lower-level management subnodes and manages a network area, assisted by a