The Class C Checker tool allows you to check whether some sites are hosted on the same Class C IP Range.

You might want to use the Class C Checker if you own several cross-linked sites. If you do, it may be more efficient (for SEO purposes) to host them on different Class C IP ranges.

Additionally, if you are going to get new links from different sites, you may want to see whether they share the same Class C IP range. Again, this could make a difference for SEO purposes.