If you know your enemies and know yourself, you will win hundred times in hundred battles. If you know yourself but not your enemies, you will suffer a defeat for every victory won. If you do not know yourself or your enemies, you will always lose.
—Sun Tzu, “The Art of War”
The goal of network security is to give people the freedom to enjoy computer networks without the fear of compromising their rights and interests. Network security therefore needs to guard networked computer systems and protect electronic data that is either stored in networked computers or transmitted in the networks. The Internet, which is built on the IP communication protocols, has become the dominant computer network technology. It interconnects millions of computers and edge networks into one immense network system. The Internet is a public network, where individuals or organizations can easily become subscribers of the Internet service by connecting their own computers and networking devices (e.g., routers and sniffers) to the Internet and paying a small subscription fee.
Because IP is a store-forward switching technology, where data is transmitted using routers controlled by other people, user A can read user B's data that goes through user A's network equipment. Likewise, user A's data transmitted in the Internet may also be read by user B. Hence, any individual or any organization may become an attacker, a target, or both. Even if one does not want to attack other ...