Subnetting is extremely tricky at first, at least it was for me. What I recommend is simply practicing the different classes (A, B, C) over and over again. Give yourself different host targets, etc. Also, ignore people who say using a subnet calculator is bad. A good sysadmin can subnet without one, yes, but as long as you know how to do it without one (if you need to) it's absolutely fine to save time and use a subnet calculator.
