Like many people here, I have more than one site, each with their own domain name. Of my six sites, I really use only one for 98% of my SEO activities. One of the six has my resume, and the others are an assortment of sites for friends and experiments that never went anywhere.
Now when I go to G and search for "related:MyMainSEOSiteURL", the results show all my six sites as well as all my previous employers' sites. It seems obvious that G's algo has somehow figured out that all the links on my resume are related. I have worked for large companies, and how they differentiate links *from* my employers to random other sites as not related is beyond me, but that's besides the point. So now that I'm ready to start other SEO sites, I prefer that they not be related in the eyes of google because if one of my sites fall out of favor or gets a penalty, I don't want the others to have the same fate. Is that a legitimite concern or does G always penalize one domain at a time? I can get the new sites for cheap if they are virtual under the same IP, is that SEO suicide? There are so many ways for G to determine a site to be related, like same analytics account, same adsense account, same registered owner... do I have to separate all those things?