You need to design a 31-door, networked Cisco Physical Access Control Gateway system. Each door will be controlled by a Cisco Access Gateway, and each Cisco Access Gateway will require its own IP address. Due to the limited number of available IP addresses, the network admin has decided to implement variable-length subnet mask (VLSM) in order to better allocate the available address space. What is the most efficient subnet mask that can be used to handle the 31 IP addresses that are needed for the Cisco Access Gateways?