What does “private browsing” really do?

When trying to protect themselves online, most people immediately gravitate to the easy to access “Incognito Mode” or “Private Mode” that most Internet browsers provide by default. While this does provide some level of privacy, it’s important to understand just how much privacy you’re getting from “Private Browsing.”

I was inspired to write this piece after a brief conversation (or three) about expectation of privacy while using work computers. When I explain to new hires that our IT dept logs their internet activity, oftentimes I’m posed with the following question:

If I use incognito mode, can you still see what I’m doing?

The short answer is yes, and most browsers actually do tell you this every time you open a new private tab. For example, here’s what Firefox tells you:

Private Browsing doesn’t make you anonymous on the Internet.

Naturally, most people don’t read this, they just open a private window and go about their business, assuming that everything they do is now secret. I even had to check if this message still shows up in current versions of the browsers that I use, since I, too, don’t actually read everything that comes across my screen.

Unfortunately, Private Browsing isn’t totally private, and to understand exactly what it does, there are a few basic concepts about using a website that you need to have a basic understanding about:

  • Browsing History
  • Bookmarks
  • Cookies
Browsing History

When you go to a website, your browser (IE, Edge, Firefox, Chrome, etc.) automatically keeps a log of all the websites you visit. This log is largely for private use, so that you can go back to look for a site you visited in the past.


Bookmarks are a useful tool to keep track of sites you regularly visit. While you can use your history to go back and look for a site, this is usually only sorted by date and not really in a organizational format that might make it easy for you to find something important. Instead, what you can do is create a bookmark and sort your bookmarks into folders to make it easier to keep your favorite or most useful sites together in case you need them. I personally keep an extensive bookmark library of tools, articles on tricky issues, and memes (useful for those group chats)


Everyone hears about cookies (and sometimes, supercookies, but we’ll talk about that later) being this crazy tool that websites can use to track you and what you’re doing, and that’s partially true. Because cookies are used by website to track you, this is most important piece to understand. The easiest analogy to a website cookie is this:

Imagine you go into a grocery store. Once you show up, you’re given a membership card with a special number that’s unique to you. After you shop around, you go to the register and pay for your things. During the transaction, the cashier scans your member card, and you go about your merry way. Scanning that card takes everything you’ve bought, and logs it.

That’s really all it is, just a special token on a website that basically creates a “history” of everything you looked at while on that site, and attaches it to that token, or cookie. When you go back to that website, the site asks for your cookie (in the background this all happens automatically). If you have one, then it just adds what it sees you doing to the log.

What cookies are used for

A lot of people I talk to often lament “I looked at cat food once on Amazon and now I keep getting ads for them everywhere! It’s like they’re tracking me!” Well, they are, and they’re using cookies to do it.

When you go to a site, that cookie is placed on your computer. Every time you come back, your computer shows the cookie to the website, and the website then chooses specific ads that they think are tailored to you based on what they’ve seen you look at in the past, and show those to you, thinking that if you see an ad for something you’re already interested in, you’re more likely to buy.

Note: I’m ignoring ad-blockers for the purposes of this post. They do various other things, but do not typically block cookies.

I’ll go into a little bit more detail on how cookies are used to identify you between different website in a different posts, but it’s important to note that this is the the most important piece of the equation.

Now that you understand what these pieces are, let’s go into what Private Browsing does and doesn’t do for you.

What it does

Every time you pull up a website, all of the above information and pieces are still created, just as it is when you aren’t in your favorite private mode. That privacy mode also does not use existing cookies and history, and when the website asks for a cookie, your computer doesn’t give it one, tricking the site into thinking you’re a new user. New history logs and cookies are created and saved to a folder on your computer. When you close the private window, your browser then looks for the history and cookies that were created there, and deletes them, so the next time you go to the website, the site doesn’t see the cookie from that session, and so it treats you as if everything you did in your favorite privacy mode never happened.

Before any of my technological colleagues roast me, there are still other methods sites use to track your activity, but that’s something we’ll cover separately.

What it doesn’t do

If you save a bookmark of a site you really liked while using private mode, or downloaded a file there, those bookmarks and downloads stick around after you’ve closed your private browser.

It’s also important to know that your computer does a lot of “talking” in the background. When you go to a website, your computer basically asks your router, “Hey, how do I get to Amazon?” Your router will then respond, “Here, I’ll take you there!” (Note: in business environments, it’s a little bit more complex, but the same principle still applies)

These communications between your computer and your router are logged on the router, which does not have a “Privacy Mode,” and at a company, your company’s IT dept can access these logs, proving that your computer was online shopping when you were supposed to be sending that report to your boss.

So what is Private Browsing good for anyway?

Seriously, if there’s all this stuff still tracking you in the background, then There are various use cases for private browsing mode, which I’ll be going over (except for hiding when you go to those sites).

Gift Shopping
If you’re shopping for gifts on a shared computer, ads that you get or your browsing history, especially with the rise of autocomplete in search engines, can give away a surprise gift for a family member before you’re ready to give away the surprise too soon. Unless someone in your home is really actively looking for the information, Private Browsing will keep the surprise a surprise, provided they don’t look over your shoulder while you’re shopping.

One-off purchases or searches
Sometimes you’re buying something really specific, or looking up something, but it doesn’t really fall under your overall interests and you don’t want it to keep creeping up in your search results or advertising, Private Browsing can partially prevent this from happening by keeping your cookies and history clear of that one time you looked up the War of the Roses.

Price Gouging and Artificial Scarcity
Airlines in particular are guilty of this practice. As you spend more and more time on an airline’s site looking at flights for that trip, or comparing prices, they’ll notice that you haven’t yet purchased the flight, so the website will then artificially raise the price every time you refresh the page, creating the illusion that that flight you were looking at is more in demand now, and if you don’t buy soon, you’ll miss out or the price will go too high.

This is a rather nefarious practice, and one of my primary personal uses for Private Browsing.

Not everyone really falls under this use case, but as an IT professional I often use private browsing to troubleshoot various issues. Sometimes I or someone I know will have trouble with a website, while their coworker sitting next to them has no problem. Cookies can sometimes become so ingrained in a website’s functionality that if they’re broken, then the website (as it appears to you) also becomes broken. With private browsing, you can test that theory before deleting all of your cookies.

I also often use private browsing to test the design of this site, or any of my public profiles. When I want to confirm what my Twitter, Facebook, LinkedIn, or this site look to a new visitor, I can open the page in a separate private window, and it will show me the site as it appears to a complete stranger.


I hope this post, while lengthy, gives you a better understanding of what Private Browsing is, and what it does and doesn’t do to protect you. It can be a valuable tool in protecting parts of your identity online, or as a tool for other reasons, but it’s not a silver bullet to internet tracking. With this information, you can arm yourself a little bit better and protect yourself on the internet.

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