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I would really love to see that! Not an easy map, given so many /24 ranges amongst the /14 and even /12 ranges. :(Well most of the approaches that I've seen with online IP checkers are delegated range lists and working down to /24s. It is not the best way for dealing with data centres because some data centres are on ISP ranges. The ISP might have the greater allocation so a /16 might end up getting blocked when in reality it could be just a /24 or less that is causing the problem.
And how would you delineate the ranges?
So let's have all the ranges and CIDRs you can find
The <If> directive, added in 2.4, replaces many things that mod_rewrite has traditionally been used to do, and you should probably look there first before resorting to mod_rewrite.