Clients, servers, IP addresses, and DNS

Most communication across the internet involves two basic roles.

One side requests information or an action, and the other side receives the request, performs work, and returns a response. The side making the request is called the client, while the side providing the capability or information is called the server.

This relationship is called the client–server model, and it appears throughout modern software.

What is a client?

A clientConcept · lights on your mapclientThe application or device that requests information or an action: your browser, a mobile app, a smart TV. A role rather than a permanent label: the same machine can be a server in a different interaction. is an application or device that requests something from another system.

Your browser is a client when it requests a webpage, and a mobile application is one when it retrieves new posts, sends a message, or submits an order.

Examples of clients include:

• A web browser requesting a webpage
• A mobile app retrieving account information
• An email app requesting new messages
• A smart television requesting a video stream
• A desktop application saving a file to online storage

The client usually begins the interaction, asking for data, submitting information, or requesting that an action be performed.

What is a server?

A serverConcept · lights on your mapserverThe software or computer that receives a request, performs work, and responds. A role rather than a permanent label: the same machine can be a client when it calls something else. is software or a computer that receives requests and provides information or capabilities in response.

A server might:

• Return a webpage
• Retrieve information from a database
• Save a new order
• Authenticate a user
• Send a photograph
• Process a search
• Stream a video

The word server comes from the idea that the system serves requests from clients.

When your browser requests a webpage, a server receives the request, performs the necessary work, and sends a response back.

This should resemble the pattern from Module 1: input → processing → output

The client sends the input, the server performs the processing, and the response becomes output returned to the client.

Client and server describe roles

Client and server are roles within a particular interaction, not permanent labels attached to certain kinds of machines.

Your laptop is normally a client while browsing the web. But it could be a server if it shared files with another device.

A large computer in a data center may be the server when responding to your phone, and that same computer may be a client when it requests information from another service.

For example, an online-store server might receive an order from your browser and then become the client when it sends a payment request to another company’s server.

A useful question is: which side is making the request, and which side is providing the response? The answer tells you which side is the client and which is the server.

Most web and mobile products use this model, although some technologies also allow devices to communicate more directly through peer-to-peer systems. You only need to recognize that exception for now.

How does a client find a server?

Before a client can send a request, it needs to know where the request should go.

Computers use numerical identifiers called IP addressesConcept · lights on your mapIP addressA numerical identifier, like 142.250.72.14, that helps networks route information toward the correct destination. Public addresses are visible to the internet; devices on a home network may share one public address via private addresses. to locate destinations across networks.

An IP address might look like this: 142.250.72.14

This is an example of an IPv4 address, while another, newer format called IPv6 uses a longer combination of numbers and letters.

But you do not need to memorize either format. The important idea is that IP addresses help networks direct information toward the correct destination.

Public and private IP addresses

Not every device has its own unique public address visible to the entire internet.

Inside your home, your phone, laptop, television, and other devices may each have a private IP address on your local network, while your router helps them share one public internet connection.

This means an IP address can identify different things depending on the network and context.

For this module, the useful mental model is: an IP address gives networks a numerical way to identify and route information toward a destination.

What is a domain name?

Numbers work well for computers, but they are difficult for people to remember.

A domain nameConcept · lights on your mapdomainA human-readable name for an online service (google.com, wikipedia.org). Registered through domain registrars and configured to direct users toward the systems that provide the site or service. is a human-readable name such as:

• google.com
• wikipedia.org
• openai.com

Organizations and individuals register domain names through domain registrars, then configure those names to direct users toward the systems that provide their websites or services.

A domain is easier to remember than a numerical IP address, but the network still needs a way to connect the name to the correct destination.

That is the role of DNS.

What is DNS?

DNSConcept · lights on your mapDNSThe Domain Name System, the internet’s phone book. It translates domain names into the information computers need to locate a service, including IP addresses. Answers are cached at several layers, and when DNS fails, healthy sites can become unreachable. stands for Domain Name System.

DNS translates human-readable domain names into information computers can use to locate the appropriate systems, including IP addresses.

It is often described as the internet’s phone book because a phone book connects a person’s name to a phone number and DNS connects a domain name to the technical information needed to reach its service. Name in, number out.

When you enter a domain into your browser, your device may ask: which IP address should I use for this domain?

DNS returns an answer, allowing the browser to continue toward the correct destination.

DNS does not always start from scratch

Your device does not necessarily perform a completely new DNS lookup every time you visit a website.

DNS answers can be temporarily saved, or cached, in several places, including:

• Your browser
• Your operating system
• Your router
• Your internet provider
• Other DNS services

If a recent answer is already available, the device can reuse it and skip a fresh request, which makes browsing faster and reduces unnecessary work.

Eventually, cached answers expire so that newer information can replace them if a website changes where its services are located.

A domain may lead to several servers

A domain name does not always point to only one machine.

Large services may use many servers across different locations, and DNS can help direct users toward different IP addresses depending on factors such as availability, location, or traffic.

This is why thinking of DNS as a simple phone book is useful but incomplete. It does more than store one permanent number for every name.

For now, the key relationship is: domain name → DNS lookup → destination information

What happens when DNS fails?

If DNS is unavailable or contains incorrect information, clients may be unable to find the systems they are trying to reach.

The website’s servers could still be running normally, but users may be unable to reach them through the domain name.

This can create symptoms such as:

• A browser saying that a site cannot be found
• Some domains working while others fail
• Applications failing to connect to remote services
• Users in one region experiencing problems while others do not

Engineers often joke that “it’s always DNS” because DNS problems can cause confusing failures far away from the DNS system itself.

But similar symptoms can have many causes, and a website failing while another internet feature works does not automatically prove that DNS is responsible. Engineers investigate before reaching that conclusion.

Tracing the beginning of a web request

You can now describe the first part of what happens after entering a website address.

First, the browser is a client that needs to find the system serving the website, so it takes the domain name you entered and obtains the necessary destination information through DNS, unless a recent answer is already cached.

Once the destination is known, the browser can begin communicating with the server.

The simplified sequence is: browser client → domain name → DNS → IP address → server

There are additional steps between discovering the address and receiving the webpage, and you will add them throughout the rest of this module.

The mental model to remember

A client requests information or an action.

A server receives requests and provides information or capabilities.

Client and server describe roles. The same machine can act as a client in one interaction and a server in another.

An IP address is a numerical identifier used to help networks route information.

A domain name is a human-readable name used to reach an online service.

DNS connects the domain name to the technical destination information the client needs.

You should now be able to explain who is asking, who is answering, and how the client begins finding the correct server.