(sorry... I had to run the code :P -- but in my own defense, I only had to run it a few times to see the pattern.)

No cigar for bmann, I'm afraid -- 192.168.0.0 thru 192.168.0.16 all pass, but 192.0.0.2 does not, nor does 192.20.0.0. (update: -- having seen bmann's update -- no, 192.168.255.255 will pass, but, e.g. 192.1.2.2 will fail.)

The key to the problem involves paying attention to the first digit in each quad, particularly in the cases where at least one quad has three digits and at least one other has fewer digits. It does make sense, and it's quite an amusing trap. Thanks.

(another update: different digit counts is not an essential property -- 100.100.100.200 also fails.)