Automatic setting of security in communication network system

A method of automatically setting one of a plurality of security methods in a communication network system having a plurality of sub-networks interconnected to one another, said method comprising:

• handling a mobile node having a care-of address dependent on a sub-network to which said mobile node is currently connected and a home address independent of said sub-network to which said mobile node is currently connected, at said mobile node:
• detecting the sub-network to which said mobile node is currently connected;
• determining the security method of said plurality of security methods corresponding to the detected sub-network from among said plurality of security methods held in a node-side security application management table to hold therein correspondences between said plurality of sub-networks and said plurality of security methods, as the security method of said plurality of security methods used for ensuring the security for user data communicated between said mobile node and a home agent associated therewith;
• and
• notifying said home agent of said sub-network to which said mobile node is connected through a mobile node network signal, and
• at said home agent:
• receiving the mobile node network signal from said mobile node;
• and
• determining the security method of said plurality of security methods corresponding to said sub-network notified from said mobile node through the mobile node network signal from among said plurality of security methods held in an agent-side security application management table to hold therein correspondences between sub-networks connected to said mobile node managed by said home agent and said plurality of security methods, as the security method of said plurality of security methods used for ensuring the security for user data communicated between said home agent and said mobile node managed thereby.