Mysql improve Self join performance large table?
Here's a stab at this: see fiddle at http://sqlfiddle.com/#!9/1fc60/1.  My algorithm in general to take all the start/end values for all clients, sort those numbers and make each adjacent pair of values into an interval.  For n clients, you have 2n-1 intervals (in worst case). Then, evaluate each of those intervals to see how many clients' IP ranges they fall into.  If more than 1, then you found a conflicting IP range between two clients.  It does works, but as you get bigger data sets, it gets considerably slower.  The algorithm is Polynomial in time complexity.  On modest hardware, I was able to process 16k clients for conflicts in about 6 minutes.

I think there's a much better algorithm that might take logarithmic time (good), or even linear time (way better), but this algo would be pretty tough to do in MySQL or any RDBMS.  It would be much easier in an OO 3GL like Java or .NET.