Network attack, means network assailants obtain the illegal authority through illegal means (decode password, electron cheat etc. for instance,), using illegal authority enable network assailant to go on host computer that attack non- operation more .Its characteristic mainly has enormous losses; Threaten the security of society and country; The means is various, the tactics are concealed; Rely mainly on the fact that the software is attacked. popular attack means as follows, utilize loophole of the network system to attack; Attack through the E-mail; Decipher and attack; the software of back door is attacked; The denial of service attacks. The precautionary measures are as follows, close the unnecessary service; Install the patch procedure; Close the useless harmful port; Delete Guest account number; Limit unnecessary user's quantity; Back up the important data in time; Use encryption system when transmitting the datas . )