A network system assists in selection of an optimum system structure for performance necessary for actual operations and a method of administrating networks. A server (222) is installed with software functioning as an SNMP agent to a monitor unit (240). An administrating server (24) has the monitor unit (240) of software functioning as an SNMP manager. A data collecting unit (300) collects structure and performance information from an infrastructure DB (26) of an administrating-assistance-subject network (2) and orders a system DB (32) to store such information. A rule generating unit (302) acquires the structure and performance information from the system DB (32), applies a learning technique of a decision tree or the like to generate sorting rules that sort the structure information in accordance with characteristic performance information at an operating time. An information extracting unit (304) acquires sorting rules from a rule memory unit (34) to provide a terminal device (36) with structure information of the other server (222) that satisfies necessary performance conditions for any one of the servers (222) at an operating time.