Being accompanied by the recently widespread Internet, computer networks are becoming an indispensable tool for communication. On the other hand, more and more devices other than computers, such as Personal Digital Assistants(PDAs) or cellular phones are connected to the Internet are as well as computers. Moreover, even cameras to take real-time images have being connected to. This tendency seems to grow stronger. Especially on the Next Generation Internet established by IPv6 networks, more and more various devices could be connected to the Internet as network devices with numerous global addresses.