An IP subnet calculator should be one of the key tools you have in your "SMS/ ConfigMgr" toolkit for those times when you need to calculate a boundary based on a non-standard subnet mask

I've always used the free WildPackets one (now renamed to the "Bitcricket IP Calculator") which is available at:

http://www.wildpackets.com/resources/free_utilities/ipsubnetcalc

Even better there is a Mac version available which I've just tried out and I'm loving it.