To provide a network security method and an apparatus for preventing unauthorized accesses in a computer network system, in which client devices and server devices cooperate to offer services through a network.

In a computer network system where client devices and server devices cooperate to offer services through a network, a server device recognizes a specific client device and notifies the client device its port accessible by the client device. Other ports of the server device except the accessible port cannot be accessed by the client device. According to this arrangement, unauthorized accesses by third parties are prevented.