Firewall port switching

A network system comprising an authorized client terminal connected to a network, a server connected to a network, and a firewall interposed between said server and said network, wherein:

• said client terminal accesses said server using publicly known protocol via a port with a publicly known port number in said firewall;
• in the case when said client terminal conducting access is authorized, said server downloads a program for realizing effective dedicated protocol solely between said client terminal and said server to said client terminal via said port having said publicly known port number;
• and
• said client terminal and said server conduct data communication via said network and said port having said publicly known port number using said dedicated protocol by executing said program,
• wherein said network system further comprises a proxy server which conducts port switching in said firewall, and wherein:
• in the case when said client terminal conducting access is authorized, said server downloads a program for realizing effective dedicated protocol solely between said client terminal and said server to said client terminal via a first port having a publicly known port number, whereafter communicates said first port to said client terminal as a port for communications, and sets a port used by said server as a second port having a port number other than said publicly known port number;
• said proxy server switches a port seen from said client terminal from said first port to said second port, and switches a port seen from said server from said second port to said first port;
• and
• said client terminal and said server conducts data communication via said network, said firewall, and said proxy server using said dedicated protocol by executing said program.