windows firewall scope syntax

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I thought the following scope syntax would be valid

IP/subnet
xxx.xxx.xxx.1/255.255.255.0

however, it keeps changing the IP to the network address

xxx.xxx.xxx.0/255.255.255.0

can anyone please explain?

i am sure xxx.xxx.xxx.1/24 works. So i can get around it, however, for cert purposes I still want to know why its doing this.
I am sure its valid and should work.