The internet network and e- commerce become more and more popular currently. Various applications of the network and services already become the indispensable important tools to most enterprises, such as the application of e mail , to establish the entry website of company, installing server to provide employees with information sharing, etc.. As the internet network providing the convenience and business opportunity , as well as e commerce be further developed, all of such IT applications created unbelievable values to enterprises. However, the security of the internet network becomes an endless issues. The external attacks , such as the electronic virus , the worm, special Lip river depends on the hobbyhorse ( Trojan Horse), procedure of back door, spy's software, the network hacker's depend event and activities have never been stopped. From which, the enterprises suffered with great losses. Therefore, the IT people of company are requested to develop and installed a suitable protection system to guarantee the security of company information assets. The case company specified in my paper is the biggest ISP in Taiwan. It owns more than three millions of customers. The company also provides its over 20,000 staffs with internal network and management network equipment for conducting routine jobs. The network and information security concerns are more complicated than that of regular commercial companies. This research will discuss the management & Network Security planning of this company from the structure and system views. Not only to create potential benefit of rigid information Security for existing network, but also to offer IT planning people with valuable reference as they are performing the related works.