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What is a CDN?

A content delivery network (CDN) is a network of servers that is geographically dispersed to enable faster web performance by locating copies of web content closer to users or facilitating delivery of dynamic content (for example, live video feeds).

Each CDN server is located on what is called the "network edge"—closer to users than the host server, which is where the website originates. For this reason, CDN servers are often called "edge servers." Each server stores or caches copies of a subset of the web content—HTML files, images, audio, video, applications—from the host server. By reducing the distance between this content and users, the content delivery network helps the website publisher provide faster performance, reduce loading time for its users and control its own bandwidth consumption and costs.

Organizations typically purchase CDN services from CDN providers, which maintain their own server networks.

In the video What Is a Content Delivery Network?, IBM chief architect Ryan Sumner goes through a scenario where a CDN helps make the website and page load time faster for globally distributed users:

For a closer look at cloud computing and edge networks, read the blog posts Cloud at the Edge and Rounding out the Edges.

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Benefits of a content delivery network

CDNs provide faster load times, reduced bandwidth consumption and many other benefits for web publishers:

Better connectivity and scalability for web publishers: A content delivery network provides site users with faster content load times. For web publishers, that equates to more page views, traffic spikes, improved customer engagement and less site abandonment.

Reduced bandwidth consumption: Web hosts charge organizations for data transferred from the origin server. By storing copies of content closer to the users, a CDN enables fewer data transfers from the origin server, reducing an organization’s bandwidth consumption and costs.

Reduced latency: Latency refers to the delay between the time data is requested from a system and when the system actually starts sending it in response. A greater distance between a user requesting web content and the server delivering it can result in greater latency. Because content delivery network servers store web content caches closer to your users, they can reduce latency and improve performance.

Better response to traffic spikes: A successful marketing campaign, a limited-time offer, a video gone viral—these types of events can create a sudden (anticipated or unanticipated) increase in content demand. Content delivery networks use load balancing to distribute this demand across servers to prevent overloading any single server. Load balancing also helps keep the spike or surge in demand from impacting website performance.

Outsourced infrastructure support: By relying on a CDN, an organization does not have to spend time, human capital or money building out and maintaining its own geographically distributed server network.

Enhanced security: Content delivery networks employ analytics and automation tools that can uncover distributed denial of service (DDoS) attacks, man-in-the-middle attacks, firewall issues and others.

Greater user satisfaction: Slow load times and issues with media playback and application responsiveness are among the chief reasons that users abandon and/or avoid websites. Working with a content delivery network can prevent or reduce some of these performance issues, making it more likely that content consumers will be satisfied with their site interactions.

Improved content delivery: Not only do CDNs deliver content faster, but they also improve the quality of the delivered content. Video replay, video calls and live video streaming can be hindered by slow transmission, which may result in jitter. Buffering, poor image and sound quality and incomplete transmissions also affect the delivery of video and audio content. Content delivery networks help by shortening the distance between the content and the user and by load balancing traffic to prevent overwhelming routers or servers.

Speedier e-commerce: E-commerce consumers have high expectations for online shopping experiences—they expect fast product image load times, quick payment method approvals and easy transactions on any mobile or desktop device. Content delivery networks help B2C and B2B retailers deliver e-commerce content and apps quickly during peak traffic periods.

How CDNs work

As previously noted, a CDN works by helping a web publisher deliver faster, higher-quality performance for users through content distribution from servers that are closer to them than the website’s origin server.

For example, suppose your website is based in the United Kingdom (UK). If someone from the United States (US) accesses your site, the CDN serves that user from an edge server in the US, closer to the user, instead of from your UK-based origin server for the web page. The result is faster content loading and web application performance, as well as improved user experience.

More than half of all web traffic is served over content delivery networks, and that percentage continues to grow as businesses expand their global reach and offer more varied content types. CDNs also distribute traffic loads so that no single server is overloaded with traffic requests. Gaming companies, cloud application creators, live-streaming on-demand media and other media services, e-commerce sites with a global reach—as digital consumption needs grow, content owners rely on CDNs to better serve their users.

CDN services

A CDN primarily offers improved web content delivery, but CDN providers offer additional services that complement serving up content.

Security services

CDNs can provide DDOS protection to data centers and websites.

In a DDoS attack, the attackers try to overwhelm a domain's DNS servers with more traffic than the can manage, with the objective of disrupting or degrading service. CDNs use analytics and automation to monitor for these attacks and respond by limiting request rates (the number of information requests an HTTP can make in a specified time period).

In an man in the middle (MITM) attack, the attacker tries to intercept or alter the communication between the origin server, CDN servers and website users. MITM attacks can occur at a number of places in a network, but CDNs help mitigate MITM attacks by adopting Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols to secure communications between the CDN and the website origin server, as well as between the CDN and the ISP.

Private CDNs

If you work with a content delivery network, you'll most likely share network functionality with other CDN customers. However, some CDNs now offer private CDNs, which provide the customer with their own dedicated CDN resources. A private CDN might appeal to an organization that has strict security needs or specific geographic requirements, or that simply wants its own dedicated edge servers that are highly available and won't suffer any latency issues.


Content delivery networks may offer real-time analytics for monitoring website traffic and gathering metrics about visitors to a site. The objective is to track user behavior. With that information, website and web application creators can optimize content for users, improve site service and target marketing efforts to specific user personas.

CDN pricing

Every content delivery network provider maintains its own pricing structure. Most charge a monthly fee based on gigabytes of data transferred from the edge servers to users. Rates vary based on the destination (the region where cached content is hosted and accessed by users). Providers also have different policies for storage; some charge storage fees while others do not.

Major CDN providers post pricing on their websites. For most providers, the per-gigabyte rate decreases as the total gigabytes of data transferred increase. Leading CDN providers also charge their customers only for the bandwidth used each month so that billing reflects actual use of the service.

Several providers even offer free levels of service. What is included in that free level of service varies widely by provider. Free and paid levels of service come with specific service level agreements (SLAs). Leading CDN providers tend to offer 99.9 percent of uptime to customers.

Before choosing a CDN provider, understand its pricing structure and SLAs. Since most providers charge based on actual bandwidth used, estimate usage before choosing a service to gain a general idea of what monthly costs will be. There are CDN pricing calculators online that help you compare pricing among top providers based on bandwidth estimates and bandwidth use by geographical region.

CDN for websites

Not every website publisher needs a CDN. A local school district's website, for example, may not need a CDN because most users will access the site from a nearby location.

But if you have a media-rich website, a geographically dispersed group of users, or mission-critical content that requires fast delivery, a CDN may be your best option.

CDN providers and hosting

The proliferation of content delivery network providers has been spurred by an increase in content types and devices used to access that content. Top providers include the following:

  • Akamai
  • MaxCDN
  • Incapsula
  • Rackspace
  • Cloudflare

When choosing a CDN provider, consider the size and distribution of its network, how well its server locations (called points of presence, or PoPs) map to the locations of your site users, customer support availability, pricing and service level agreements (SLAs). Also, consider whether the provider offers any additional services that would be helpful to your organization, such as added security and analytics services.

Content delivery network hosting describes the networked servers of a CDN provider that host selected web content from a website. While website hosting typically refers to only one server, CDN hosting includes many servers networked together. CDN hosting augments website hosting by caching content in network servers that are geographically closer to website users. This differs from a web server, which hosts your full site on the origin server. CDN hosting can, therefore, deliver content to users faster than the website's origin server.

Open source CDNs

Not every organization can justify the cost of working with a content delivery network. Open source CDNs provide a less costly, although more time- and labor-intensive, option. With open source CDNs, you can link to libraries of content, such as CSS or JavaScript frameworks. Open source CDNs host elements of website infrastructure on CDN servers. Website content managers can access that content for free. Open source CDNs do not host your website’s original content. However, they can improve content delivery by moving common web structural elements used by your site closer to your users.


For website operators with robust content storage needs, content delivery network providers offer storage clusters that integrate with their network of edge servers. Website operators may want this storage capability if they serve large static files, such as videos or installation files. By storing these files closer to the user, CDN storage delivers better service and faster downloads. These storage options also relieve the traffic burden on the origin server by decreasing load requests and routing those requests to CDN edge servers instead.

Learn more about how you can leverage cloud object storage across all IBM CDN offerings.

CDN tutorials

If you're ready to learn more about using content delivery networks, try one of these tutorials:

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Resources What is load balancing?

Load balancing lets you evenly distribute network traffic to prevent failure caused by overloading a particular resource.

What is a DDoS attack?

DDoS attacks flood websites with malicious traffic, making applications and other services unavailable to legitimate users.

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