When I add new device in OpManager by device name/IP address why I cannot choose Netmask to be all 255(255.255.255.255).

The last mask that I can chose is 255.255.255.252. But, if I have loopback interface with netmask all 255 and want to add device by that address it is not possible because netmask 255.255.255.255 not exists in netmask list.