说明：  上课时老师用的课件，非常详细。适合0基础人士学习网络安全。想学习网络安全，必须掌握良好的理论基础。然后就是去应用，比如可以通过思科的模拟器，模拟练习网络的互连。
(The courseware used by the teacher in class is very detailed. It is suitable for 0 basic people to learn network security. If you want to learn network security, you must master a good theoretical basis. Then it is to apply, for example, through Cisco's simulator to simulate the interconnection of the network.)