Network security is now days becoming more and more important because people like to connect with each other all the time via internet. Personal computer users, employees of professional organizations, government servants, academicians, social workers, students, military peoples etc are very familiar to use network currently and all these people use the available network for most of their work. All these people keep their most important data on internet and also do the money related online transactions. The internet structure is itself such that there may be possibility of threats to occur. To secure our network we must have to know which type of security threats may occur and how? By knowing this we may able to find out security methods against these threats.