A communication control in a communication network system using a mobile computer, capable of realizing flexible address control and management for the mobile computer. When the mobile computer is located within a home network, a communication with the mobile computer is carried out by transferring data using a home address specific to the mobile computer which is a private address. When the mobile computer is located within an external own organization network, a communication with the mobile computer is carried out by routing data to the external own organization network using a care-of address indicating a current visited location of the mobile computer in the system which is a global address, and addressing the mobile computer within the external own organization network using the home address. When the mobile computer is located within an external other organization network, a communication with the mobile computer is carried out by routing data to the external other organization network using the care-of address, and addressing the mobile computer within the external other organization network using a mobile address assigned to the mobile computer at a time of moving outside the own organization networks which is a global address.