I just developed this since I have on multiple occasions wanted to see if an IP address was part of a range block. I've been unable to find anything like this as part of the MediaWiki software so I hacked one up:
http://layla.chrishowie.com/~chris/checkrange.php
For testing purposes, this is an IP that's part of the Scibaby range block:
http://layla.chrishowie.com/~chris/checkrange.php?ip=76.64.38.92
Comments welcome.