Why do you say that? Google's interest is in making sure that links between sites are legitimate.
If (as you intimate) that one of those tests involves looking at the third octet of the IP on which two websites are hosted on then you're also saying that Google is relying on classful addressing - that is, that websites hosted on different class Cs are on separate networks. This is not the case and hasn't been the case since the mid 90s.
I can't see Google relying on an outdated routing/addressing methodology (that was outdated when Google started) to determine whether sites are related.
The only people I've generally seen talking about class C addresses are low rate SEO companies.