A /31 IP address is a very small subnet that is typically used in point-to-point links between two devices, such as routers or switches. It consists of only two usable IP addresses, one for each end of the link, as the first and last addresses in the subnet are reserved.
In the past, a /30 subnet was used for point-to-point links, but due to the scarcity of IPv4 addresses, network administrators started using /31 subnets to conserve IP addresses.
The use of /31 subnets is supported by some operating systems and networking equipment but not all, so it's important to check compatibility before using them.
Overall, a /31 IP address is used when you need a very small subnet for a point-to-point link and want to conserve IP addresses.