Hi clark_white,
You can't decide to start a range at an arbitrary point - the mask affects where you must place the start.
For /24s, you can start at anything that ends in .0 - e.g. 10.21.42.0/24
For every bit that you take from the mask, you must make the start of the mask an even multiple of 2 to the power of that number; e.g.
/23s must have an even number (divisible by 2) as the network address. e.g. 10.21.42.0/23 is the start, but 10.21.43.0 can't be the start of a /23
/22s must have a multiple of 4 as the network address. e.g. 10.21.42.0 can't be the start of a /22; it must be 10.21.40.0/22 or 10.21.44.0/22
/21s must have a multiple of 8 as the network address. e.g. 10.21.44.0 can't be the start of a /21; it must be 10.21.40.0/21 or 10.21.48.0/21
... and so on with /20, /19, etc. up to the next octet boundary.
This is something that's worth studying until you can nail it every time. I found the Cisco CCNA official certification guide by Wendell Odom a really good resource on this, but i'm sure HP have something as well. There are plenty of free resources on the Internet: just search for "learn to subnet" or something like that. Plenty of videos may be found on Youtube as well.
Good luck!