Linking on the same C Class IP Block
Linking on the same C Class IP Block is considered to be highly devalued compared to links from other IP addresses. This is because it is quite obvious that a lot of links all coming from the same C Class IP Block is interlinking between several web site all hosted on the same server and hosting account.
This type of linking doesn't look natural and the search engines know this. Search engines check which IP address a URL is at and thus know where all related links are coming from. A few years ago it was possible to create several sites and link them to one main site and get a boost in rankings. That does not work any more.
What is a Class C IP Address?
OK, so what exactly is a C Class IP Address? The internet Digest has a good article on IP Address classification.
Basically, for SEO purposes when we talk about the same C Class IP block we mean that we are talking about web sites that are hosted on the same server and usually the same hosting account.
It is important that linking appears natural to search engines. It is highly unlikely that 90% of your links come from the same server unless it is artifically influenced.
The SEO Files Recomends
Build links naturally and make sure that your links come from a variety of sources rather than the same server. Don't create web sites as linking platforms for a main site.