First of all, you need to know what is a C Class IP. When you look at an IP address, it contains 4 sets of numbers in the order "111.222.CCC.444". The third set of numbers are known as C Class of the IP address. Since search engines want to avoid spamming and cross linking by the webmasters they evaluate the links on the basis of the C Class IP associated with them. In order to ensure that you are getting relevant back links that will be well received by the search engines as well you must get them from different C Class IP address.
In order to identify this property, you can use several IP tools available as browser extensions. Once you are able to track the IP you'll be able to simplify the decision. Just make sure that you can get links from sites with different C Class IPs, these can be article directories, press release sites, web directories as well as social bookmarking sites. It might be any source of getting a link back but make sure that the C Class parameter is different.
Over a period of time you will realize that the impact that this factor has on your rankings is enormous. You'll be able to get better rankings on major search engines including Google. Another great tip for webmaster who have multiple sites is to host them with different providers so that when they cross link they come from different C Class sources which will have a positive impact on your site.