Video streaming with HDTV or UHDV quality will be provided and widely demanded in the future. However, the transmission bit-rate of high-quality video streaming is quite large, so generated traffic flows will cause link congestion. Therefore, when providing streaming services of rich content, it is important to flatten the link utilization, i.e., reduce the maximum link utilization. To achieve this goal, parallel video streaming in which ISPs use multiple servers to deliver rich content is effective. However, the effect of parallel video streaming depends on the network topology and link capacities. In this paper, we investigate the impact of network topologies on the effect of parallel video streaming using 23 actual commercial ISP networks, when optimally designing server locations and optimally selecting servers.