Provided is a network system including a management device and one or more external networks which connect each of any two of computer systems, wherein the management device acquires communication performance of each of the external networks among a bandwidth, a delay, and a maximum transferable packet, receives a calculation request for the communication performance of a virtual network, specifies a configuration of the requested virtual network, calculates the communication packet of the specified virtual network based on the acquired communication performance of the external network, and provides information of the calculated communication performance of the virtual network to a manager.