They are all yours, under your control. For reasons we'll go into later, a good choice would be to use some number beginning with 10, but you can choose something else if you insist. Your choice for that depends on how many machines will be on this network. It is possible that you may have been assigned a number or range of numbers and a mask by someone else. If so, just go with the flow for now and pretend that isn't true.
Every machine needs a unique number.
So, if I were setting up the very first machine, I might assign it 10.1.1.30, and then the next 10.1.1.31, and so on. What I can't do is decide that machine number 4 will be 11.1.1.34. And, if I used the subnet mask of 255.255.255.0, I can't assign the next machine 10.1.4.30 either. The reason for that has to do with that mysterious subnet mask, and this is where it gets confusing. 10.2.32.98 172.16.15.15 192.168.223.223 Remember the 10.1.1.30 was just one choice we could have made.