In this paper, we propose a simple but effective VPN mechanism called RING-VPN (Ring-based Virtual Private Network) that realizes a high scalability in terms of the number of VPNs. The key idea of our RING-VPN is to logically connect nodes in a ring topology for minimizing the number of IPsec tunnels. In our RING-VPN, each VPN node operates autonomously, making VPNs robust even in case of node and/or link failures. We also quantitatively evaluate the performance of our RING-VPN using mathematical analysis. We derive several important performance metrics of RING-VPN, such as VPN construction time, and VPN recovery time, as well as user-level performance metrics, such as minimum TCP throughput, round-trip time and packet loss probability. Furthermore, we validate our analysis by comparing numerical examples with simulation results. Through several numerical examples, we quantitatively demonstrate effectiveness of our RING-VPN in several network configurations.