Ip & seo
I have around 20 websites, all at the same IP. I have one main domain, and 19 subdomains. I backlink from most of them to my main (web design) site, since these are sites I created for other people, the links are relative. I rank pretty high for my niches in most paces on Google, and so do some of my clients ( the ones who work on their content, and seo). Seo is definitely more about the web page content, than IPs and subdomains. Bottom line, search engines are interested in returning the most relevant results to searchers. GIve them some meat, and you will find yourself moving upward.
Have a blessed day.