Just on this, and I'm surprised it hasn't been mentioned yet, is to get the correct IP addresses of those sites that are nestled behind the likes of CloudFlare infrastructure. It will break your IP addresses, and give them all the same value.
Fortunately they have some server headers available too.
Instead of me rewriting what's already been written, have a look here for a more concise answer, and yes, I went through this process a long while ago too.
https://stackoverflow.com/a/14985633/1190051
