A network system characterized by comprising: an Ethernet communication device; an upper-level monitoring device positioned between an outside Internet and the Ethernet communication device; a firewall positioned between the outside Internet and the upper-level monitoring device and being configured to permit checking compatibility or suitability of data through an authentication process of the system; and at least one protection relay configured to: determine a process of data received through the Ethernet communication device, store a first security level of a plurality of systems and a second security level of a plurality of source addresses, check a security level of a system among the plurality of systems that transmits the data based on the first security level and check a security level of a source address among the plurality of source addresses that is included in the data based on the second security level prior to and during processing the data to permit rechecking compatibility or suitability of data that have passed the firewall through an authentication process of the source address, and generate a signal including information instructing whether to short-circuit power based on results of the checking, wherein the received data includes at least information related to monitoring or controlling the at least one protection relay or information related to monitoring or controlling an upper-level host of the at least one protection relay, wherein the security level of the system is determined based on use or non-use of the information related to monitoring or controlling the upper-level host, and wherein the first security level uses an IP address and a subnet address of the upper-level host, and the second security level uses an MAC address of the upper-level host. an Ethernet communication device; an upper-level monitoring device positioned between an outside Internet and the Ethernet communication device; a firewall positioned between the outside Internet and the upper-level monitoring device and being configured to permit checking compatibility or suitability of data through an authentication process of the system; and at least one protection relay configured to: determine a process of data received through the Ethernet communication device, store a first security level of a plurality of systems and a second security level of a plurality of source addresses, check a security level of a system among the plurality of systems that transmits the data based on the first security level and check a security level of a source address among the plurality of source addresses that is included in the data based on the second security level prior to and during processing the data to permit rechecking compatibility or suitability of data that have passed the firewall through an authentication process of the source address, and generate a signal including information instructing whether to short-circuit power based on results of the checking, determine a process of data received through the Ethernet communication device, store a first security level of a plurality of systems and a second security level of a plurality of source addresses, check a security level of a system among the plurality of systems that transmits the data based on the first security level and check a security level of a source address among the plurality of source addresses that is included in the data based on the second security level prior to and during processing the data to permit rechecking compatibility or suitability of data that have passed the firewall through an authentication process of the source address, and generate a signal including information instructing whether to short-circuit power based on results of the checking, wherein the received data includes at least information related to monitoring or controlling the at least one protection relay or information related to monitoring or controlling an upper-level host of the at least one protection relay, wherein the security level of the system is determined based on use or non-use of the information related to monitoring or controlling the upper-level host, and wherein the first security level uses an IP address and a subnet address of the upper-level host, and the second security level uses an MAC address of the upper-level host.