Google Chrome has a "do not track" option, but with a warning:

Many websites will still collect and use your browsing data — for example to improve security, to provide content, services, ads and recommendations on their websites and to generate reporting statistics.

Does this mean websites know which other websites I visited? Like if I visit a website about cats, the site admin knows which dog websites I previously visited?

I had no idea they could do that. I thought the only ones that know all the sites I visited are Google, ISP, and the manufacturer of my PC.

For example I thought "cookie" was just to track info from my last visit to the same website that gave me the cookie.

3 Answers 3


Depends entirely on what cookies they use. If they only set cookies from their own domain (first-party cookies), they might be able to know that you visited their site before, but it wouldn't tell them anything about third party websites, in general.

If they set cookies from other sites (third party cookies), the third party might be able to tell that you have visited example.com, example.org, and example.net - but only if all of these sites include third party cookies from the same provider. Example.com wouldn't be able to tell that you'd also visited example.org, unless the third party told them (perhaps through an online dashboard), or you followed a link from example.org to example.com, and your browser set the "referrer" header when connecting.

Most third party cookies belong to large advertising companies, such as Google or Facebook, or to social network companies, such as Twitter or Facebook. Because these companies are included on a lot of sites for various reasons (enabling sharing functions, analytics monitoring, embedded feeds of data), they can easily build up a picture of where you have been.

When you opt out of tracking (the "do not track" option) on any of these sites, it just sets another cookie which tells them not to link any data about your online behaviour together.

Your ISP knows what sites you visit through different means - your data passes through their systems, so they can just grab out a list of where you have been. Depending on your country, it might be a legal requirement for them to store this data.

Your PC manufacturer, though, probably can't tell what websites you've been on, unless they pre-installed some tracking software (look up "Lenovo superfish" for an example of this), but this is not particularly common, since it causes all kinds of bad publicity for them, and doesn't provide them with any useful information.

  • "Do not track" does even less than setting a cookie - it can be ignored by default or often interpreted counterintuitively. Possibly in the question, "PC manufacturer" means "Microsoft", and the default Windows setting "enable page prediction, which sends my browsing history to Microsoft" in Windows 8 and 10 Internet Explorer / Edge browsers, and Cortana, could be relevant. Commented Nov 10, 2015 at 18:19

There are several mechanism how a website learns about other websites visited.

Most common is the referrer header, telling the target website where you are coming from. This is of special interest to advertisers who know exactly which ad was working for them. They also learn about your interests.

If you are using Google Chrome, Google learn about the websites you visit because Chrome sends information to the Google headquarters.


Websites know which other websites you visited, because your webbrowser knows and changes its behaviour based on your browsing history.

If you have visited a website before, the browser styles links to the visited websites differently (compared to non-visited websites). By 'reading' the difference in style, a website showing a link to another website can figure out if you have visited the linked website before.

I'm not entirely up-to-date as whether or not the this is considered a bug or security issue. Years ago, it was considered a privacy issue, but more recently I've observed the behaviour again.

  • Most browsers actively try to prevent this nowadays, although there are almost always bugs in the behaviour allowing for edge cases. Specifically, scripts will usually be unable to determine whether a link is in "visited" state, and some CSS properties which could be detected implicitly (e.g. font-size) just get ignored. See hacks.mozilla.org/2010/03/… for some details on this in Firefox, but IE and Chrome are similar.
    – Matthew
    Commented Nov 11, 2015 at 11:33

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