I’m sure I missed something somewhere and I’ve been going thru my book trying to find it
My Boson labs often give me an ip address and tell me to assign it. In the solution, there will be thr accompanying subnet mask that you’re supposed to put with it. I know how yo subnet, I know about Network Bits, Subnet Bits, and Host Bits. I know the formula.
Like how does 12.5.10.0 get you a subnet mask of 255.255.255.0?
When you are given an IP address to assign to a particular host or interface, the subnet mask should accompany that address so you can configure it appropriately. However, as @nitayp1 has correctly stated, if no subnet mask is provided, then in most cases it is assumed that you use the classful subnet mask.
The only way you would get such a subnet mask for this IP address is if it has been explicitly stated. If it has not been explicitly stated, and the mask is not classful, then I would have to conclude that a mistake has been made in the book or documentation you are reading.