A mobile node moves from a first IP (Internet Protocol) network to a second IP
network in a network system in which the first IP network capable of executing
communication in accordance with both first and second kinds of IPs and the second
IP network capable of executing communication in accordance with only the first
kind of IP are connected with each other. When the mobile node communicates a message
with other nodes on the first network after its movement accordance with the second
kind of IP, a header for the movement containing both home and foreign addresses
of the first kind in IP is added to a header containing home and foreign addresses
in the second kind of IP, and put to the message, is added. The message to which
the movement header is thus added is used for the communication between a first
mobile agent on the first network and a second mobile agent on the second network,
or between the mobile node and the first mobile agent.
