What Is a CDN, and How Do CDNs Work?

CDN stands for content delivery network. They are crucial for a solid user experience on the internet. They have considerable power with respect to improving site loading speed and the reliability of site delivery.

If you don’t know what a CDN is, you might be confused as to what we are talking about. Don’t worry; all confusion will be addressed soon.

Let’s start from the top.

What is a CDN?

A CDN is a network technology, meaning it is related to the internet. To understand CDNs, we must first understand DNS.

DNS stands for Domain Name System. The DNS is a system of virtual servers all over the world that stores information about domains. Domains, or domain names, are human-friendly names that are used to identify websites on the internet.

Common examples include Google.com, Facebook.com, and Microsoft.com. A domain is the human-friendly part of a website’s identification. It is easy to remember.

Computers, on the other hand, require more information than just a domain name to identify a website. They need an IP address. An IP address is a series of numbers (IPv4) or hex digits (IPv6) that tell a computer exactly where the server of a website is.

So, what does DNS have to do with all of this? It is the DNS’ job to maintain a directory that maps all domain names to IP addresses. Without the DNS, no computer would ever know which IP address belongs to which domain.

Now, how do CDNs come into all of this? CDNs are content servers that are distributed globally. They store a cached version of the site that the client is trying to access. The CDN physically closest to the client will respond to the client’s request and provide them with the website. This means that when a website is being served to the client, there is less latency due to the reduced distance, and the site loads way faster.

What are the components of a CDN?

CDNs have several components that come together to provide the complete function. Generally, they include the following things:

PoPs (Points of Presence) or Edge Servers

Origin Servers

DNS Servers

Let’s see what each thing is.

PoPs

PoPs or edge servers are physical servers that have either HDDs, SSDs, or RAM installed that store cached versions of a website. These are placed in various places in the world. This means there is a high chance that one of these edge servers is in relatively close proximity to the client.

Origin Servers

These are the original servers that hold the actual website data. They are the ones that the client is actually trying to reach. However, they can be very far from the client in terms of distance. This is why they rely on edge servers to provide a lower latency experience.

DNS Servers

Of course, without DNS servers, it is impossible to know which server the website wants to visit. However, with CDNs involved, DNS servers have another role to serve. They are responsible for providing the closest edge server’s IP address to the client instead of the origin server.

This is done by creating CNAME records and A/AAAA records. If you don’t know what records are, they are the actual files that have the domain to IP translation data. They are stored in DNS servers. The A and AAAA records have the IP information, while the CNAME record helps to alias other domains (such as sub-domains to the main one). In the CDN’s case, CNAME records are used to alias the PoPs to the origin server.

The hosting provider–the company that sells server space to domain owners to host their websites–provides CDNs as an additional option. This includes the whole setup of origin servers, PoPs, and the required DNS infrastructure to make the system work.

How Do CDNs Work?

Now that we know about the components of a CDN let’s see how they work.

Look at Figure 1. You will notice that there are three types of entities: The Origin Server, PoPs, and the clients.

In this figure, you can see that the clients are physically closer to the PoPs than the origin server.

So, when they make a DNS request for the origin server’s IP address, the DNS system instead provides the IP address of the closest PoP. The client connects to the PoP, which serves them a cached version of the website.

This transaction takes less than a few seconds because of the close proximity and reduced latency.

So, how does the DNS resolver know that it has to divert clients to the closest PoP? Well, a simple way to do it is to point your root domain (the origin server’s domain) to the CDN-provided subdomain.

You do it by changing the A/AAAA records to provide the IP address of the CDN. The IP ranges available in a CDN belong to the edge servers. Since one domain can have multiple IP addresses, it is possible to point toward all PoPs/edge servers by adding multiple A/AAAA records. Additionally, if you have subdomains for your website, then you can point their CNAME records to the CDN-provided subdomains as well.

This way, all DNS requests for your domain (i.e., the origin server) will be automatically diverted to the CDN (the PoPs).

One issue that can arise here is accidents in record updating. Since all these records have to be added and updated manually, human error can occur. It is advised that you must check whether your records are published correctly using a resource such as DNS Checker–a free and reliable tool.

As for using the closest server? That’s up to the networking protocols. They have built-in parameters that enable clients to find the closest server. So, this functionality, paired with CDNs, allows clients to connect to the closest edge server/PoP.

Conclusion

CDNs are a necessity for webmasters and online businesses in this day and age. They enable online businesses and service providers to provide better and more reliable services to their customers.