What Is a CDN?

A content delivery network (CDN) is a distributed network of servers that caches and delivers web content from locations near users to speed up websites and applications. A CDN places copies of content on edge servers spread across many geographic locations, so a request reaches a nearby server instead of a distant origin. Providers such as Cloudflare, Akamai, Fastly, and Amazon CloudFront operate global CDNs with thousands of points of presence.

This article defines a content delivery network, explains how a CDN works through edge servers and caching, sets out its benefits, describes what content a CDN caches, and lists the major providers. Each section states one part of the topic and connects it to the distributed edge servers and caching at the center of the definition. The result is a complete account of what a CDN is, how it delivers content from near users, and why websites and applications rely on it for speed and reliability.

What Is a CDN?

A content delivery network (CDN) is a geographically distributed network of edge servers that cache and deliver web content from a location near each user. A CDN stores copies of content on servers around the world, so a user retrieves data from a nearby edge server rather than from a single distant origin server. The defining traits of a CDN are listed below:

• Distributed edge servers place content copies in many geographic locations close to users.
• Caching stores copies of content so repeated requests serve from the edge without reaching the origin.
• Request routing directs each user to the nearest or fastest available edge server.
• An origin server holds the original content that edge servers copy and cache.

A content delivery network reduces the distance data travels, lowering the delay measured as network latency. The edge servers carry content using the protocol suite described in the overview of TCP/IP.

How Does a CDN Work?

A CDN works by caching content on edge servers in points of presence worldwide and routing each user request to the nearest server holding a copy. A CDN intercepts requests for content and serves them from a nearby cache, fetching from the origin only when the edge server lacks a current copy. The steps a CDN follows are described below:

1. A user requests content, and the CDN’s routing directs the request to the nearest edge server.
2. The edge server checks its cache for a current copy of the requested content.
3. On a cache hit, the edge server returns the cached content directly to the user.
4. On a cache miss, the edge server fetches the content from the origin, stores it, and returns it.

A point of presence (PoP) is a physical location where a CDN places edge servers, often near internet exchange points. Request routing relies on the address resolution and addressing defined in the overview of an IP address to send each user to a nearby PoP.

What Is a Point of Presence (PoP)?

A point of presence (PoP) is a physical data center location where a CDN houses edge servers to cache and serve content close to a region’s users. A point of presence brings content geographically closer to users, reducing the distance each request travels. The traits of a point of presence are listed below:

• Geographic placement positions each PoP near population centers and internet exchange points.
• Edge servers within a PoP cache content and respond to nearby user requests.
• Many PoPs across regions let a CDN serve a global audience with low delay.
• Origin offload happens because PoPs answer most requests, reducing traffic to the origin.

The number and spread of points of presence determine how close a CDN can place content to any user, which directly affects network latency. Major providers operate hundreds or thousands of PoPs across the global network.

What Content Does a CDN Cache?

A CDN caches static content such as images, stylesheets, scripts, and videos, and can also accelerate dynamic content that changes per request. A CDN stores content that many users request unchanged, while applying separate techniques to content generated individually. The content a CDN handles is listed below:

• Static content includes images, CSS stylesheets, JavaScript files, fonts, and downloadable files.
• Video and streaming content is cached and segmented for delivery near each viewer.
• Dynamic content, generated per request, is accelerated through optimized routing and connection reuse.
• Cache control headers tell the CDN how long to store each item before refreshing it from the origin.

Static content caches easily because it is identical for every user, while dynamic content requires techniques such as route optimization and persistent connections to the origin. Cache control headers, defined in the HTTP specification, set how long an edge server keeps each cached item before checking the origin for a newer version.

What Are the Benefits of a CDN?

A CDN reduces latency, lowers origin server load, improves reliability, and adds protection against distributed denial-of-service attacks. A CDN improves both the speed users experience and the resilience of the service behind it. The benefits of a CDN are listed below:

• Lower latency results because content travels a shorter distance from a nearby edge server.
• Reduced origin load follows because edge servers answer most requests instead of the origin.
• Higher reliability comes from many servers, so one failure does not stop content delivery.
• DDoS protection absorbs and filters large traffic floods across the distributed network.

Lower latency is the primary benefit, since shorter distance reduces the round-trip delay measured as network latency. Distributed denial-of-service protection works because a large CDN absorbs attack traffic across many points of presence rather than at a single origin.

Who Are the Major CDN Providers?

Major CDN providers include Cloudflare, Akamai, Fastly, and Amazon CloudFront, each operating a global network of edge servers. A CDN provider runs the infrastructure of points of presence and routing that delivers content near users. The major CDN providers are listed below:

• Cloudflare operates a large global network combining content delivery with security services.
• Akamai runs one of the oldest and most distributed CDNs, with edge servers across many networks.
• Fastly provides a CDN focused on configurable edge logic and fast cache control.
• Amazon CloudFront integrates a CDN with Amazon Web Services for content stored in that cloud.

Each provider operates points of presence worldwide, differing in network size, edge features, and integration with other services. Amazon CloudFront connects to the wider set of services described in the overview of cloud networking.

How Does a CDN Differ From an Origin Server?

An origin server holds the original, authoritative copy of content, while a CDN’s edge servers hold cached copies distributed near users. The origin is the single source of truth, and the CDN distributes copies of its content to reduce distance and load. The differences are listed below:

• The origin server stores the original content and updates it when the website changes.
• Edge servers store cached copies that expire and refresh from the origin on a schedule.
• The origin handles only requests the edge cannot answer, reducing its total load.
• Edge servers handle the majority of requests, serving most users without reaching the origin.

A CDN does not replace the origin server but sits in front of it, caching and delivering its content from locations near users. The origin and edge servers communicate over the same protocol suite described in the overview of TCP/IP.

What Security Features Does a CDN Provide?

A CDN provides security features including TLS encryption, distributed denial-of-service mitigation, web application firewalls, and bot management at the edge. A CDN sits between users and the origin, so it filters and protects traffic before it reaches the protected server. The security features a CDN provides are listed below:

• TLS encryption secures connections between users and edge servers using HTTPS.
• DDoS mitigation absorbs and filters large traffic floods across the distributed network.
• A web application firewall (WAF) inspects requests at the edge and blocks malicious patterns.
• Bot management distinguishes automated traffic from legitimate users and limits abusive requests.

A CDN’s distributed scale lets it absorb distributed denial-of-service attacks that would overwhelm a single origin server. Filtering at the edge inspects each request before it reaches the origin, applying rules based on the destination service and its network ports.

What Is the Difference Between Push and Pull CDNs?

A pull CDN fetches content from the origin on the first request and caches it, while a push CDN requires the operator to upload content to the edge servers in advance. The two methods differ in when and how content reaches the edge servers, which affects setup effort and control over caching. The differences are listed below:

• A pull CDN retrieves content from the origin automatically the first time a user requests it.
• A pull CDN simplifies setup, since the operator changes only the origin and the edge fetches updates.
• A push CDN requires the operator to upload content to the edge servers before users request it.
• A push CDN suits large files served infrequently, giving the operator direct control over what is stored.

A pull CDN suits frequently updated websites because the edge servers fetch new content on demand, while a push CDN suits large static assets such as software downloads. Both methods reduce the distance content travels and lower the network latency a user experiences.

Key Takeaways

• A CDN is a distributed network of edge servers that cache and deliver content near users.
• It works by caching content in points of presence and routing requests to the nearest server.
• A point of presence (PoP) is a location housing edge servers close to a region’s users.
• A CDN caches static content directly and accelerates dynamic content through routing.
• Benefits include lower latency, reduced origin load, higher reliability, and DDoS protection.
• Major providers include Cloudflare, Akamai, Fastly, and Amazon CloudFront.

Last Thoughts on CDNs

A content delivery network is a distributed network of edge servers that cache and deliver web content from locations near users, reducing the distance each request travels. A CDN works by caching content in points of presence worldwide and routing requests to the nearest server, caching static content directly while accelerating dynamic content through optimized routing.

The benefits include lower latency, reduced origin load, higher reliability, and protection against distributed denial-of-service attacks, delivered by providers such as Cloudflare, Akamai, Fastly, and Amazon CloudFront. Readers can continue with the overview of network latency, the guide to cloud networking, the overview of TCP/IP, or the guide to how networks work.