Let's say I have a site that sells something. A user has no idea that my site exists, but they visit a competitor website which adds a cookie to their browser. Can I display advertising for my site to this user on the web based on that cookie? Via an ad network for example?

I realize it seems like a simple thing, but I just want to confirm that this type of targeting is not restricted to RE-targeting, where I show users an ad based on a cookie that came from MY site.

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You cannot read cookies from other sites on your site, so you would have no way of doing this with generic cookies set on some other 3rd party domain.

It would be technically possible if both your site and the other site used the same ad network. Then the ad network would be able to set a common cookie for the visitor and recognize them in both places. Then ad network would have to enable this functionality, allowing advertisers to show ads based on which other sites they had been on. However, I don't know any ad networks that enable such functionality. Not many webmasters would use ad networks that allowed their competitors to target ads like that.

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