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Are Content Delivery Networks (CDN) Good or Bad for SEO?
by Dr. William Sen | CEO and founder of blue media

Content Delivery Networks (CDN): Good or Bad for SEO?

What is a CDN and does it harm SEO - Especially for Images

If you ever asked yourself if using a Content Delivery Network is bad for SEO, you might have heard lots of different opinions. A lot of articles mention the load speed of a page. But it’s not just that. Anyway, we have lots of things to talk about CDNs, particularly its SEO advantages and disadvantages, so let’s dive in.

First off, I would like to clarify what CDNs are and how they are used. So, if you want to skip the introductory part, jump to the main articles:

What is a Content Delivery Network (CDN)?

A CDN is an external service provider besides your web server, which hosts your files such as videos, software, java scripts, and many others. When embedded in your CMS, the media files, for example, are being displayed from an external source, instead from your website. Here is an example how to use CDNs for images:

Let’s say you have an article under the URL

Instead of having the hero image hosted by that URL

…you would have it hosted under a CDN like this

CDNs do not host your whole content or website but just help in caching of page components such images or videos, to accelerate the speed at which they load. The CDN has data centers that can be anywhere on the globe.

Why do people use Content Delivery Networks (CDNs)?

Old news: Load speed matters for SEO. The slower your website, the lower your rank.

Once a website gets bigger and the sessions increase, sometimes all they need to do is move the web server to a faster web server host: either to a co-location, to self-owned server infrastructures or nowadays to a scalable cloud computing platform such as Amazon Web Services (AWS). This seems to be the first step to provide a better and faster web experience.

But when your website becomes a giant, traffic and speed once again become a challenge. Especially when you have to deliver millions of files such as images through your website every day, you will face severe symptoms of slowdowns. It starts with compromised bandwidth and server speed. Not long later, it will affect user experience, and eventually: Google will find out that you can’t serve your users as intended and will rank your website down.

While more sessions are considered an SEO heaven, at the same time the slowdown it causes is the SEOs worst nightmare. Welcome to the SEO vicious circle!

Why use a CDN?

The main reason to use CDNs is to increase the speed and load time of web pages. Quite simply solve the annoying delay. Especially websites such as news organizations that have millions of images, have to face a huge amount of data traffic caused by images, articles and also other media files such as videos.

Imagine for a moment you have millions of sessions on your website every day (it’s a wonderful thought, I know): However, in this case, even images can cause a heavy traffic and burden your bandwidth a lot. Of course, without having a CDN, you could still try to overcome this challenge by installing load balancers to your server ecosystem and redesign the database infrastructure – on top of that you might have to extend the number of servers, and have to constantly think about getting more bandwidth.

And that’s when CDNs come in pretty handy.

You don’t have to worry about all that anymore – instead, you would outsource all your bigger files such as even illustrations,  images and videos to an external host that is specialized to manage the fast delivery of large media files. It’s much faster, but also much cheaper than to scale your web server infrastructure and to maintain it.

It seems to be a good solution, but there is an SEO flipside to this, but more about that later in this article. First, let’s have a look at how CDNs actually work:

How does a Content Delivery Network (CDN) work?

Most of the CMS nowadays, even WordPress, have the appropriate plugins to handle CDNs. Once in alignment with your CMS, the image handling, for example, looks like nothing has changed: the media file is uploaded through the CMS such as WordPress as usual. But the media files are automatically diverted to the CDN, and at the same time, the CMS makes sure that the media file is being loaded from that CDN. That way the editors wouldn’t even know about the existence of a CDN because on the CMS backend, everything looks as is. But as an SEO, when you look at the source code of a web page, you will see that the image is not loaded from your website’s URL, but from another one.

When you’re using CDNs for your images, for example, all your images will be delivered from that CDN. That will enormously increase the speed of your page load as the CDN is specialized in delivering large data files extremely fast. As CDNs have to manage large files, they use high bandwidths, provide scalable server infrastructures with all the gimmicks that come with it: load balancers, state-of-the-art and efficiently configured Database Management Systems (DMBS).

The SEO Advantages of CDNs

Once a CDN takes care of the delivery of your larger files such as images, the amount of traffic your page is causing will be limited to HTML/text only. This will make each of your pages extremely small from a page load speed and traffic perspective. It’s the perfect SEO for page speed.

Unfortunately, that’s the only SEO advantage you’ll get from this.

As page speed seems to be the only benefit for SEO when it comes to CDNs, it’s important to know when actually using a CDN makes sense.

One of the rules in engaging a CDN is when it comes to larger video files. Using a CDN to increase your page speed for the video file is a good idea. Google doesn’t really index video files unless they live on YouTube. So, if you have a website with lots of embedded video files on the same web server, you might want to think about using CDNs instead, rather than hosting the video files directly under the same server.

But when it comes to images… well, that’s the problem:

The SEO Disadvantages of CDNs

Most of the articles about SEO and CDN are about page load speed. That’s nice, but all of them seem to miss the most important disadvantage: Images!

In my experience, more than 95% of the SEOs are missing the opportunities of Search Engine Optimized Images. In my article “How to use SEO Optimized Images” I stressed out the importance of unique images for pages. And in the past and today, using unique images for SEO turned out to be one of the most effective SEO methods. You might want to read the part where I mention “Image Optimization is a blessing for Smaller Campaigns” in my article.

If your website is the Times Magazine, you don’t have to care about SEO optimized images – in that case, you couldn’t care less about the fact that you are using a CDN to host your images. As your website is already one of the top-ranking sites on the World Wide Web, why would you be concerned about image optimization, right?

And this is what it comes down to: If you host your images throughout another web server or URL, you give away the Google trust for that image to another source. As the image physically lies somewhere else, it’s not necessarily a part of your website from a Google standpoint. All the positive effect you can benefit from in my article about “Image Optimization Article” will have no meaning for your website if you use a CDN. That’s a fact.

Google’s Standpoint on CDN for Images

You won’t find Google’s standpoint on how to utilize CDNs when it comes to images. The reason most likely is Google would rather prefer that you use faster servers such as CDNs to fulfill the user experience, rather than having a slower load speed.

In a sense, it benefits Google when you use CDNs. But would Google try to understand that the image that is being displayed in another URL belongs to your website? Why would it bother?

All Google cares about is user experience, and not how well you are going to be ranked.

Google may or may not crawl these images just to increase their image portfolio for their image search, but won’t care much about how these two elements (your URL and the CDN URL) are connected. For all Google knows, that image even doesn’t belong to your website.

Long story short: You won’t get any SEO credits for that image you’re using, no matter how great you think the image is or how well related it is to your content.

I’m not saying, it’s a bad thing. Google might rank your page anyway, and even on the highest positions on the SERP (Search Engine Result Pages). But it’s just the content Google is taking in consideration now, not the image anymore. In conclusion:

You’re missing all the nice opportunities when you don’t have your image under the same URL as your website.


If you have to use CDNs for whatever reason, you should at least consider using a subdomain. Many CDNs provide the ability to create a subdomain for your domain, where the images can live under. By changing DNS records, you can your propagate a designated subdomain such as images.<yourdomain>.com to the Content Delivery Network. That way all your images or any other media files will look like they live in your domain ecosystem.

It’s a good start, but when it comes to subdomains, it’s never safe to say that Google will acknowledge your subdomain as a part of your website. Especially in this case, as only images would live under that subdomain, it’s most likely Google will just see that subdomain as a file archive, and an entity not related to your website. It might, or it might not.

Our research indicates that Google handles giant websites such as Times Magazine and others differently than the other ones that are in the billion. Google uses so-called focused crawlers when it comes to giant websites that act as the gatekeepers of the web. Don’t expect the same treatment for your web project, unless the Google CEO knows who you are.

We’ve researched a couple smaller website projects that have been using CDNs and could see that they were not getting any SEO credits for their images – many of them couldn’t even be found through Google Image Search. Some of the websites we checked were also using subdomains, hoping that would make a change – but it would not.

To sum up, if you are not a giant website having challenged high traffic and slow load speed, don’t use CDNs for images at all. In other words, it is best to keep your images under the same URL if you want to profit from Image SEO, unless your website has hundreds of thousands of images it can’t handle.

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CATEGORIES Search Engine Optimization
About the Author
ABOUT THE AUTHOR Dr. William Sen CEO and founder of blue media

William Sen has been an SEO since 2001 and is a Software Engineer since 1996, and has been working as an Associate Professor in Germany for the University of Dusseldorf and Cologne. He has been involved in developing custom SEO tools, large website and software projects. William has a PhD in Information Sciences and has been working for brands such as Expedia, Pricewaterhouse Coopers, Bayer, Ford, T-Mobile and many more. He is the founder of blue media.

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