Suppose we have point-to-point connection between routers R1 and R2. Since there are always one client when sending packet, we don't need broadcast address in this situation. We also need 2 address to assigned to R1 and R2 interfaces in the network. And we need a network address to identify this point-to-point network. That concludes in total 3 addresses are needed. However in /31 subnet mask only maximum 2 address can be provided. How come /31 subnet mask is possible if 3 addresses are needed and only maximum 2 addresses can be provided?