Abstract: This paper briefly describes the security structure of the computer network, and the service problem of the Internet in the NC-CSB (National Center of China Seismological Bureau). This NC-CSB network has been divided into the local network and external network by router and firew all service. The local network access Internet through Proxy service. AC-CSB IBM firew all software was installed in IBM RS/6000 43P workstation. It has 3 Ethernet Adapters. One is for local network; the others are for external network. NC-CSB has two Internet ports; their supports in the Internet services follow packet filtering by firewall for the network security.

Key words: Network structure, Network security, Network service, Packet filtering