Blacklist module: Full IPs are blocked, but not ranges

See this screenshot:

http://www.screencast.com/t/LDiMiarx

When I test it on my own IP, e.g. 64.55.128.121, it works if I add that to the blacklist.

But when I enter a range, as specified in the docs, such as 64.55.0, it doesn’t block. Everything gets through.

For testing, I try to use my contact form both before and after the blacklist setting. I full IP blacklist entry results in the contact form failing submission, as expected.

When I change the blacklist to an IP range, it allows a submission to go through (obviously defeating the purpose)

What am I doing wrong?

Paul