Operating a cluster on a single IP address is required when the cluster is used to provide certain Internet services. This paper proposes SAPS, a new method to assign a single IP address to a cluster. The TCP/IP protocol is handled at a single node called the I/O server. The other nodes, called application nodes, provide the socket interface to applications. The I/O server and applications nodes are connected using a cluster-dedicated network, such as the Myrinet network. The key benefit of the proposed method is that the TCP/IP protocol does not care about congestion and packet loss in the cluster, which often happens if multiple nodes send packets to the bottleneck router. Instead, the clusterdedicated network manages the packet congestion more efficiently than the TCP/IP protocol. The result of the bandwidth benchmark shows SAPS fully utilizes the bandwidth of the Gigabit Ethernet. The result of the SPEC Web benchmark shows SAPS handles 7.9% more requests than the existing method...