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Targeted Advertisements on the Internet

Author: Michael E. Duffy
August, 2019 Issue

We’ve all had the experience of visiting a website about, say, the best patio furniture to buy, and then seeing advertisements for patio furniture as we visit other websites. I recently visited the Wealthfront website ( to learn more about their cash account, which currently pays 2.51 percent. Now I see ads for Wealthfront when I’m on Facebook and other sites that display ads. Most people find ads that follow them around the Internet either magical or creepy.

Of course, it’s not magic. It’s technology. It just means that your Internet browser accepts “cookies” when you visit a web page. A cookie is just some data given to your browser by the web server that delivered that page to your browser. Whenever you visit that web page, the browser sends back all the cookies that the web server gave it last time.

Cookies date back to the earliest days of the browser (1994) to allow data to persist between visits to a website. In particular, to support a virtual shopping cart (i.e. remembering items a visitor wanted to purchase between visits to an e-commerce site). Back then, a cookie might contain a lot of information (e.g. all the items in your shopping cart). But nowadays, most cookies contain an identifier that uniquely identifies you. The rest of the information about you is stored in a database, and accessed using that identifier.

You’ve probably heard of cookies. When encountering problems with a website, one frequent recommendation is to “clear your cookies,” and try again. Clearing your cookies means that your browser forgets any information stored by a particular website. You can also avoid cookies using your browser’s “incognito” or “private browsing” mode. In the past, you could avoid limits on the number of free articles you could read at the The New York Times, Washington Post, or other media sites, but many now detect incognito/private modes and refuse service.

A web server can only retrieve its own cookies when it serves up a page of a particular website. But if the content of that web page includes information served up by another web server, that web server can also exchange cookies with your browser. So, if web server A shows an ad that is served up by web server B (a common occurrence), your browser will return cookies for web server A to server A, and cookies for web server B to server B. If it’s the first time server B has encountered you, it will give you a unique identifier, which will now be revealed on any web page that serves up information from server B, following you around the Internet.

Curious about what cookies you’re receiving from a given web page? In Chrome, click on the icon to the left of the page URL in the address bar (an “i” in a circle, or a lock symbol). For example, if you visit and click on the lock symbol, it shows that it has given me 86 (!!) different cookies, many of them from advertising networks. The cookies that belong to are called “first party” cookies. The others are benignly referred to as “third-party” cookies.

In reality, these third-party cookies form the basis for targeted Internet advertising. And the information on a page coming from a web server in an ad network doesn’t even have to be an ad. It can be an invisible one-pixel image, or any URL from that ad server. All that is required in that the web site you visited has an agreement with the ad network to embed the necessary information in their web pages.
You can tell your browser not to accept third-party cookies (search Google for “block third party cookies” along with the name of your browser). You’ll still see advertisements, but they won’t be targeted based on other pages you’ve visited. You can regularly delete your browser cookies (an estimated three in 10 users do so). Or, you can add an ad-blocking extension to your browser.

As of February 2018, Google Chrome began blocking all ads from sites which display any ad that violates certain guidelines (like automatically playing sound). Apple Safari does so as well. Both these efforts are in hopes of keeping users from getting full ad-blocking software such as AdBlock Plus, AdBlock (no relation to Plus), or uBlock Origin. Of course, since lots of useful sites depend on advertising, it’s possible that wide-spread use of full ad blockers would cause these sites to curtail or cease operation.

So, when ad seems to follow you around the Internet, it’s not magic. Is it creepy? Once you understand how it works, it’s decidedly less creepy.  But third-party cookies do allow for fine-grained tracking of your online habits.

I’m not particularly bothered by this because I know the free sites I frequent depend on them for their livelihoods. Since I don’t use ad-blocking software, I’d rather have ads that are targeted to me than not. And sometimes I even discover topics to write about here.




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