From the basic telecom service to the IDC service, the telecom carriers have realized the transformation from the telecom hardware products to the telecom integrated services. The core network design is an important component of the construction for IDC. Based on several important principles of building a core system for IDC, in this paper, the key points of the design of core network architecture are put forward, around the four parts of the Internet access layer, the core layer, the server access layer and the operation management platform. The applications of firewall technology, virtual network congestion control and energy saving technology are also prospected from the future development of IDC and the key demand of customers. Through the good design of the core network, the telecom carriers can improve the core competitiveness of the enterprise and realize the sustainable and healthy development of IDC business.