Lesson
Hosts, Clients and Servers
Learning objective
Recognize hosts and explain how applications can act as clients, servers, or both.
What makes a device a host?
In lesson 1, your laptop created a print job and the printer received it. Both were end devices. In networking, we call these devices hosts.
A host is an end device that participates in network communication. It is a source or destination of data, rather than a device whose job is to connect other devices or networks.
The shape or price of a device does not decide whether it is a host. Its participation in the network does. A printer with a network interface is a host.
Types of hosts
Meet the hosts you will see throughout this course. Notice what each one can do on a network, and become familiar with its icon.
Desktop or workstation
A fixed computer used at a desk for work, study, or specialist applications.
Laptop
A portable computer that can join wired or wireless networks.
Phone or tablet
A mobile host that uses applications and network services while moving between networks.
Network printer
A host that receives print jobs through its own network interface.
IP camera
A camera that sends video or images through a network connection.
IoT device
A sensor, appliance, or controller that exchanges small amounts of useful data.
Physical server
A dedicated computer that provides services to other hosts.
Cloud or virtual server
A software-defined server that is still a host even when its physical machine is hidden from the user.
Network interfaces
A host joins a network through a network interface: the hardware and software connection point it uses to send and receive network data. Think of it as the host's doorway to the network.
What is a network interface card (NIC)?
A network interface card (NIC), also called a network adapter, is the hardware that gives a host a physical connection to a network. Despite the name, it does not have to be a separate card: it can be built into a laptop or phone, installed as an expansion card, or connected as a USB adapter.
Host ↔ NIC / network adapter ↔ Network
Data travels in both directions. This is a simplified view of the adapter's role, not a separate host between your computer and the network.
What does it do? It sends the host's network data onto a connection and receives incoming data for the host to process. An Ethernet adapter uses a cable; a Wi-Fi adapter uses radio signals. These connections let the host use services such as websites, shared files, and network printing.
Where might you use one? A desktop may use a built-in Ethernet adapter to connect to a switch. A laptop may use its built-in Wi-Fi adapter to connect to an access point, or a USB Ethernet adapter when it has no Ethernet socket.
The adapter is the hardware; the interface is the connection point the host uses. We will cover its addressing and detailed data handling in later lessons.
Wired: Ethernet
The laptop's Ethernet interface connects to a switch through a cable.
Wireless: Wi-Fi
The laptop's Wi-Fi interface connects by radio to a wireless access point.
These are separate examples. A laptop can have both interfaces; it does not need to use both to join a network.
An interface is a connection point, not another host. Detailed addressing and link behaviour come later in the course.
Clients and servers
Now that a host is connected, what does it do? Suppose you open a website. Your browser asks for a page, and a web server application provides it.
A client application asks for a service. A server application provides that service and responds.
Requests a service
Client → Server: request a page
Server → Client: return the page
Provides the service
This shows application roles, not the physical cable path. Network devices between the hosts are omitted.
Client and server describe what applications are doing in one conversation. They are not permanent hardware categories: a large computer is not automatically a server, and a phone is not always only a client.
Follow host conversations
Watch who asks, who provides, and how the response returns. Focus on the role each host has in the current conversation.
Follow a host conversation
Choose an example, then follow who asks for a service and who responds.
A browser requests a page and a web server responds.
Active: Laptop
Packet inspector
Inside the packet
Host or device decision
No frame is crossing a link during this step.
- Starting host
- Meet the laptop
One host, more than one role
One host can run several applications at once. The same computer can run a client application and serve a file to another device. Its role depends on what each application is doing in that conversation.
In the file-sharing player, Computer A first requests a file from Computer B. The next conversation reverses those roles. The computers did not change type; their applications changed jobs.
Classify host roles
Select each example to decide whether the application is acting as a client, a server, or both.
Classify the host role
Client and server roles describe what applications are doing, not a permanent type of machine.