Jim Boykin creates another free SEO tool. The reports take about 3 minutes to make. It tells unique linking domains, unique linking IP address, and unique linking C blocks (as well as how many links from each unique linking domain).

Since links from sites on the same C blocks are more likely to be owned by the same person some search relevancy algorithms may lower the weight given by links hosted on the same C block.

Try the WeBuildPages C Class Backlink Analyzer Tool

here is another cool C class IP address tools I mentioned in the past:
IP Report