The transmission bandwidth consumed by delivering rich content, such as movie files, is enormous, so it is urgent for ISPs to design an efficient delivery system minimizing the amount of network resources consumed. To serve users rich content economically and efficiently, an ISP itself should provide servers with huge storage capacities at a limited number of locations within its network. Therefore, we have investigated the content deployment method and the content delivery process that are desirable for this ISP-operated content delivery network (CDN). We have also proposed an optimum cache server allocation method for an ISP-operated CDN. In this paper, we investigate the properties of the topological locations of nodes at which cache placement is effective using 31 network topologies of actual ISPs. We also classify the 31 networks into two types and evaluate the optimum cache count in each network type.