As a case study, in October 2009 Google update, I have about 5 sites. And get two of them are sitting in the same server, and connected to each other. In general all of my sites is to increase the PageRank, but two are on the same server.
The main problem is, I found these two sites, the homepage/index page PageRank lower than inside. I did a reverse check reverse, all that will be the main page, instead of inside pages. The question is, why get better PageRank inside it?
We know, from A to B when the web page PageRank, flow together. PageRank must come from the inner index pages, because there is no external source to link to inside pages.
Finally, I found a site administrator from the Internet forums, there is discussion of the past few years. Its conclusions are the same IP address is the main cause of this. Some people think that it is the Google sandbox. But it is not entirely correct. Google sandbox is your site is dropped entirely from the Google SERP.
Sure that you either need to be placed in different sites on the Web server in a different location, or to get a C class for your IP address for each site.
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