A communication device carries out a routing operation of data between a first network and a second network. The communication device comprises: a first interface connected to the first network; a second interface connected to the second network; and a processor that assigns an IP address to a user terminal device connected to the first interface in accordance with a DHCP. The processor changes a first lease period of the IP address set when the processor assigns the IP address to the user terminal device in accordance with the change of a network environment.