In SEO, C Class IP is important as it is related to back links. When Google and other search engines are indexing the back links to your site, they want to see back links from a variety of IP address (URL’s). If all your back links come from the same IP (URL) address, it doesn’t look natural and the back links will loose their value.
Example: If you are doing directory submission and you submit 10 directories, out of which 9 directories have same c class IP, then search engine only consider 2 directories out of 10.
Also when Google notices that all your back links are coming from the same IP address, it considers this activity suspicious as if you were trying to manipulate the system.
In the era of spam, many webmasters have started creating multiple website on same server and then interlink these websites to increase the sites back link and ranking. Search Engines have identified this issue and started avoiding links coming from similar IPs. So when Search Engines see that you’ve a link coming from same C Class IP on which your website is located, the back link will not have any importance. Please note you’ll not get penalized if you’ve link coming from same C class IP, but you’ll not get any advantage either. So if you are doing link exchange especially if you don’t have your own server, do check if the person you are linking to has a different C class IP than yours.