[pbhjpbhj]

You work at Google? IP address isn't used at all in the ranking algorithm? Not even to identify "voting rings" (sites passing link-juice that normally wouldn't, eg a site made only for that purpose)?

Can I ask why not? IP seems like a really useful signal.

[yellowapple]

Not a Google employee, but I reckon ranking down sites with shared IP addresses would disproportionately impact sites on shared servers regardless of the reason. Plenty of websites out there rely on cheap/free hosting providers that shove everything behind an Apache or nginx instance with a single address. This is especially common/useful now that SNI is mainstream and widely supported.