1. An IP-address distribution system comprising:
a network formed by a plurality of switching devices; and
a plurality of IP-address distribution devices connected to the network, wherein the switching devices are grouped into two or more groups, wherein at least one of the IP-address distribution devices is connected to a sub-network, which is a network formed by switching devices in a same group, and
a switching device, among the switching devices, having a physical port connected to another switching device in another group is configured to interrupt, at the physical port, an IP-address request signal, transmitted to the plurality of IP-address distribution devices from a terminal included in a first group that requests allocation of an IP address from the plurality of IP-address distribution devices, wherein the interruption includes blocking passage of the IP-address request signal between a logical port of a first switching device in the first group and a logical port of a second switching device in a second group and preventing the IP-address request signal from reaching at least one of the plurality of IP-address distribution devices which is connected to a sub-network of the second group that is different from the first group.