Abstract: A computer system to which measures against computer virus can be taken with ease, and a method therefor. In a computer network system, a first GW server is assigned an IP address to which a netmask of 24 bits (allowing to perform communication with 256 other nodes at the maximum) is imparted, and is allowed to perform communication with all of client computers. Upon occurrence of an abnormal condition such as viral infection, the client computers are assigned or reassigned IP addresses including a netmask (for example, 30 bits; allowing to perform communication with two other nodes at the maximum) that only allow to perform communication with a security measure server via a second GW server, whereby the client computers are allowed to perform communication only with the security measure server via the second GW server. In this way, the security measures are taken.

Abstract: According to the present invention, there is provided a computer system to which measures against computer virus can be taken with ease, and a method therefor. In a computer network system (1), a first GW server (28) is assigned an IP address to which a netmask of 24 bits (allowing to perform communication with 256 other nodes at the maximum) is imparted, and is allowed to perform communication with all of client computers (10). Upon occurrence of an abnormal condition such as viral infection, the client computers (10) are assigned or reassigned IP addresses including a netmask (for example, 30 bits; allowing to perform communication with two other nodes at the maximum) that only allow to perform communication with a security measure server (4) via a second GW server (3), whereby the client computers (10) are allowed to perform communication only with the security measure server (4) via the second GW server (3). In this way, the security measures are taken.