Trying to give something back to the Community...
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:
Even better there is a Mac version available which I've just tried out and I'm loving it.
Read the complete post at http://wmug.co.uk/blogs/cliffs_blog/archive/2011/05/13/downloads-looking-for-a-free-ip-subnet-calculator.aspx