Google Ranking Factor – Network Neighbourhood – Links
Links from other websites in your network neighbourhood and also within your IP C-Block accumulate less credibility for ranking credit.
Websites on the same server, or websites within the same hosting provider network may be related in some way such that the link is possibly unnatural.
A classic example of unnatural backlinks is where a web developer builds many websites for its clients and then places links back to themselves on each website they built. The sites are most likely within the same C Block and maybe on the same IP.
The site that is negatively affected by same IP or IP C-Block is the site which receives the link. Interlinking within IP and IP C-Block also gains little if any credit toward rank, especially sites in different genres as such sites would probably have no reasonable cause to link to each other to benefit their visitors and is probably only done for a misplaced SEO strategy.