Good guides for those that need a quick dive:
Wikipedia: CIDR notation. Terse. Without understanding of IP addressing and subnets worthless. So the following give a better big picture:
Cisco: IP Addressing and Subnetting for New Users
MediaWiki's guide to blocking ranges: Help:Range blocks
Yahoo! Small Business Help: What is CIDR?
Microsoft: Understanding TCP/IP addressing and subnetting basics
I also wrote a CIDR->netmask converter function in bash for use with vpnc:
http://bit.ly/ARld
$(()) for math is pretty handy.
Damn that's pretty cool Mat.