According to Google on the subject of shared IP addresses:
Lots of sites are hosted on shared IPs. If this had a negative effect on ranking, it would harm most of the sites on the web--and that's not good for small webmasters or for our users. So, understandably, sharing an IP should not have an effect your ability to rank.
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[Source: A Small Business Blog and Small Business Tips Related to Growing, Marketing and Managing a Small Business.]
