Content Delivery Network (CDN): What It Is & How It Works

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What is a content delivery network (CDN), how does it work, and what are the pros and cons of CDNs when it comes to SEO? Learn more here.

Content Delivery Network (CDN): What It Is & How It Works

With Google’s Page Experiment looming right around the corner (it’s set to start rolling out in June), site developers and business owners worldwide are funneling more time and resources into user experience improvements.

Given the upcoming update’s focus on loading, interactivity, and visual stability, it’s an opportune time for site owners to audit their site performance.

Perhaps you’ve heard that one major facet of this should be a reevaluation of the site’s web hosting and content delivery network (CDN) set up.

In this post, you’ll learn what a CDN is, how it works, and why CDNs matter for SEO.

Why You Need a CDN

The average internet user isn’t concerned with how webpages work or what web hosting provider a company uses.

All they care about is their own user experience – mainly fast loading speed, quality content, and intuitive navigation.

Speed equals money, and this is especially true for ecommerce sites. In fact, pages that load within 0 to 2 seconds have the highest ecommerce conversion rates.

Moreover, conversion rates decrease by an average of 4.42% with each additional second of loading time between 0 and 5 seconds.

Even for non-ecommerce sites, page loading speeds drive the bottom line by influencing bounce rates; those that load in 1 second have an average bounce rate of 7%.

This compares to an 11% bounce rate for pages with a 3-second page latency and 38% bounce for pages that take 5 seconds to load.

Across industries, using CDNs is a standard strategy for achieving optimal page loading speeds for both desktop and mobile. Cisco research shows that global CDNs are expected to carry 72% of all internet traffic by the year 2022.

It’s all but impossible now to meet user expectations and contend with competitor sites without using CDNs.

What Is a CDN and How Does It Work?

Websites that don’t use CDNs rely on a single server to distribute content to all site visitors, whether they’re a mile down the road or located across the globe.

On the other hand, a content delivery network consists of multiple geographically distributed servers.

CDN providers often place servers at internet exchange points (IXPs), the physical location where internet service providers connect and exchange traffic.

Site owners pay for CDN services, allowing them to distribute website content like images, videos, audio, HTML, CSS, and JavaScript files to each server in the network.

These servers store cached versions of the website content, making it readily available for user requests.

When a user requests a page, the content is delivered via the server that’s closest geographically. In turn, CDNs reduce page latency by minimizing the distance a site’s content must travel when requested by an end-user.

CDNs also improve loading speed and user experience by optimizing delivery based on the type of content requested, such as standard web content, dynamic content, video streaming, or large file downloads.

Aside from improving loading speed, using CDNs increases bandwidth and lowers overhead costs for servers.

Why CDNs Matter for SEO

User Experience Metrics

It’s high time for SEO professionals and business owners to make loading speed a top priority.

User experience and SEO are innately intertwined, and Google has long considered UX elements to determine search rankings.

For example, the 2015 Google Mobile-Friendly update transformed the SEO arena by introducing mobile-friendliness as an “on or off” algorithm.

In July 2018, Google confirmed that landing page loading speed is a search ranking factor, including for mobile searches.

Per Google’s algorithm announcement, the May 2021 update will make user experience even more important.

Many previous algorithm updates have taken site owners by surprise and have forced SEO pros to conduct large-scale research to pinpoint and analyze the changes.

But this time, Google is telling website owners exactly which metrics to monitor and improve.

The web.dev website provides an overview of all the Core Web Vitals and has a tool for testing your site.

Regarding loading speed, the site provides concrete figures for the ideal LCP and FID speed and CLS visual stability. Optimal performance metrics are under 2.5 seconds, under 100 milliseconds, and under 0.1, respectively.

Advanced Analytics

Premium CDN providers also include analytics and insight reports as a part of the package.

CDNs can collect and report critical information such as audience analytics, geographic and query-based traffic data, quality of service data, security event analytics, and viewer diagnostics.

CDN packages often offer customizable insight dashboards that make monitoring these performance indicators easy and automatic.

Site Security

Using a CDN also protects sites against denial-of-service (DDoS) attacks. Because CDNs distribute content to numerous servers, it prevents DDoS attacks from affecting the original server.

Moreover, if a server within the network is attacked or experiences more traffic than it can handle, the request will reroute to another server.

Website security indirectly affects SEO by improving user experience and building trust in the site or brand.

Potential Pitfalls of CDNs for SEO

Image Hosting

While the SEO benefits of using CDNs are clear, the potential disadvantages are a bit murkier.

For example, well-known figures in the SEO industry have questioned the impact of using CDNs on image rankings.

A 2019 article by Barry Schwartz analyzes Twitter interactions between prominent SEO Lily Ray and Google Senior Webmaster Trends Analyst John Mueller. When Ray asked how to retain SEO value for images hosted on a CDN subdomain, Mueller replied, “It doesn’t matter how you host the images, there’s no SEO-bonus for having them on the same host/domain.”

This statement from Mueller contradicted a widely held belief within the SEO industry that hosting images on a CDN domain – even a subdomain of your website – can hurt an image’s performance.

This point of view is encapsulated in the popular article Content Delivery Networks (CDN): Good or Bad for SEO? from William Sen, the CEO and founder of blue media.

In the article, Sen contends that using CDNs counteracts image optimization efforts, even if you host them on a subdomain:

“…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. Google decides based on its own algorithm if it will consider your subdomain a part of your site or not – Google handles subdomains differently based on each website.”

The article goes on to explain that Google uses focused crawlers to crawl and index high-profile gatekeeper sites, giving them “special treatment” and closer analysis than the average website.

Whether you agree with Sen or take Mueller’s Tweets at face value, the best thing you can do to leverage image SEO is still to set up your CDN with a custom subdomain that correlates with our primary domain.

Ahrefs recommends setting up a CNAME record or alias to clean up your CDN subdomain name.

Ahrefs also recommends recovering link equity by reaching out to sites that use your images but link to the image source (the CDN) or the image itself instead of your site.

You can use Ahrefs’ Site Explorer tool to identify links that point to images hosted on the CDN. Simply reach out to the publisher and request they change the image URL to your domain.

Duplicate Content

Another pitfall to avoid when using a CDN is the potential for duplicate content. However, if you set up your CDN correctly, you shouldn’t have any issues.

Brian Jackson from KeyCDN says you can work around this issue by using a canonical header setup that tells Google crawlers that content on the CDN is a copy of the original.

Preparing for the User Experience Update

If you currently don’t use a CDN or are unsatisfied with your current provider, consider making the switch before the User Experience Update rolls out in May.

Every time Google launches a new core algorithm update, there are “winners” and “losers” among the most-impacted sites.

The dramatic traffic drops that many sites faced after the Medic Update serve as an example of how an algorithm change can undo months of SEO efforts overnight.

Fortunately, Google decided to give webmasters notice of the upcoming update. Google never has and likely never will make their exact search algorithms public.

However, the User Experience Update announcement, identification of Core Web Vitals, and resources provided on the web.dev site show an unprecedented level of transparency for Google.

This time around, SEO professionals know what’s coming and can prepare for the update by improving UX and measuring their efforts against the performance metrics listed as Core Web Vitals.

Improving page loading speed through the use of a powerful CDN, among other UX and SEO strategies, should help prepare your site for upcoming algorithm updates.


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