IPs can be shared as well as dedicated. A shared IP address implies that there are a host of other web sites that are using the same IPs address on the same server. However, a dedicated IP implies that that there is only one site with the same IP address on a server. So which one is better for SEO?
BruceClay blog targets at giving a solution. It says, “We recommend that any site serious about their rankings spend the money (it's only a few extra bucks a month) to be on a dedicated IP.”
It further signifies the importance of a dedicated IP address by giving proof of it through a research:
It’s important for a few reasons. First, we’re pretty sure it plays at least a small part in the search engine’s algorithms. The engines will use reverse lookup to determine what kind of IP you’re on. If you think they don’t care, consider this: It’s been said that about 3 percent of all Web sites have dedicated IPs, with the other 97 percent resting on shared IPs. Research was then conducted analyzing the top 50 results for certain queries in the various search engines. The research found that 90 percent of the top-50 results were using dedicated IP numbers.
One of the reasons using a dedicated IP can have a positive influence on rankings is because the engines take into consideration how fast your site loads in comparison to other sites. If you’re sharing your IP with 500 other Web sites, the server, like all good customer services departments, will deliver the files in the order they received them.
For a better insight read the complete article.