Heretofore, in an IP (Internet Protocol) v4 network, a terminal device has assigned an IPv4 address thereto from a DHCP (Dynamic Host Configuration Protocol) server. In this environment, a DHCP packet has been monitored and captured by a firewall. In the case where the terminal device makes communication by using an address other than the IPv4 address assigned thereto from the DHCP server, then the IPv4 packet concerned is blocked by the firewall.
For example, in the following Patent Literature 1, it is described that an IPv4 packet of which IPv4 address is not the IPv4 address assigned from the DHCP server is not allowed on communication by a switching hub. Moreover, as another example, in the following Patent Literature 2, it is described that the IPv4 packet of which IPv4 address is not the IPv4 address assigned from the DHCP server is regulated by a network apparatus.