Lesson 1 — What Is a Network?

Week 1Lesson 1Required15 min estimated0% progress

What Is a Network?

Understand what a network is, which devices connect endpoints, and how local traffic differs from remote traffic — before diving into frames, addresses, and protocols.

What you'll learn (6 objectives)~15 min: video → lesson → check → apply → lab prep

Learning objectives

Define network, LAN, WAN, client, server, and endpoint in plain language
Identify the primary role of a hub, switch, router, firewall, and wireless access point
Explain why switches replaced hubs, and tell a collision domain from a broadcast domain
Describe CSMA/CD as historical Ethernet collision handling
Define PDU before encountering the term in diagrams or tools
Trace a simple local vs remote communication path and name the default gateway's role

Terms you will see

NetworkLANWANClientServerEndpointSwitchRouterHubFirewallWireless access pointDefault gatewayPhysical topologyPDUCollision domainBroadcast domainCSMA/CD

Time breakdown

Read the notes7 min
Device explorer3 min
Apply activity5 min

Opens on YouTube

Network Devices (Day 1)

By Jeremy's IT Lab · Opens externally on YouTube

Watch time not yet confirmed for this video — the notes below cover everything this lesson requires.

Watch on YouTube

The link opens in a new browser tab. Return here when you finish watching.

Watch for these concepts

Difference between endpoints, switches, and routers
What a LAN is versus the Internet or a WAN
Why a router sits at the boundary between networks
Where a firewall inspects traffic
How a wireless access point extends a wired LAN

Go beyond the video

The assigned video introduces the idea. CCNA Practice Labs completes the learning path: clarify core concepts, explore how each device behaves, visualize the communication path, check your understanding, and prepare for hands-on lab work.

Watch the concept → understand it → visualize it → practice it → apply it.

What makes something a network?

A network is a group of devices that can exchange data using agreed rules. Those devices might be in one room or across continents. What matters is that they share a communication path and understand the same protocols.

A LAN (local area network) connects devices in a limited area — an office, classroom, or home.
A WAN (wide area network) connects LANs across distance — branch offices, ISP links, or the Internet backbone.
The Internet is a global network of networks. Your home LAN reaches it through a router and your ISP.

Endpoint

An endpoint is any device that sends or receives application traffic — a laptop, phone, printer, or server. Switches and routers forward traffic; endpoints create and consume it.

Clients and servers

A client requests a service. A server provides it. Your browser is a client when it asks a web server for a page. The same physical device can act as a client in one moment and as a server in another — a file share on your PC makes it a server to other clients on the LAN.

Clients initiate conversations (HTTP request, DNS query, email send).
Servers listen and respond (web page, DNS answer, mailbox storage).
Both need a network interface — wired NIC, wireless radio, or virtual interface — to place bits on the wire or air.

The basic network devices

CCNA expects you to recognize each device by what it examines and what it connects. Use the Network Device Explorer below to compare them interactively.

CCNA Practice Labs · Interactive

Network Device Explorer

First-party

Client (endpoint)

Requests services from other devices

Examines: Application needs (URL, file name, email address)
Connects: Endpoints on the LAN or across the Internet
Example: A laptop opening https://example.com in a browser

Following one communication path

Local traffic stays inside one local network. Remote traffic crosses a router (often behind a firewall) to reach another network or the Internet.

Default gateway

The default gateway is the router interface a device sends traffic to when the destination is not on its own local network. A device only knows how to reach its immediate neighbours; for anything further away it hands the traffic to its gateway and lets the router work out the rest. When your laptop reaches a website, its first stop is always the default gateway.

Two views of the same network are worth separating early. The physical topology is how devices are actually connected — which cable or wireless link runs from each device to each port. The logical communication path is the route data takes from sender to receiver. They often differ: your laptop may physically connect to an access point in the ceiling, while logically it is simply talking to a server two hops away.

Local path

Client → AP / Switch → Server

Remote path

Client → Switch → Router / Firewall → Internet → Server

Local: Laptop → access point or switch → server on the same local network.
Remote: Laptop → switch → router/firewall (the default gateway) → ISP → Internet → remote server.
Switches forward based on MAC addresses within one local network.
Routers forward based on IP addresses between different networks.

A first look at what travels across the network

PDU — define before you diagram

A Protocol Data Unit, or PDU, is the name given to data as it is handled at a particular networking layer. You will see “PDU” on exam diagrams and in Packet Tracer — it always means “the unit of data at this layer,” not a mystery acronym.

Application data — what the user or app creates (web page, DNS name, file chunk).
TCP segment or UDP datagram — transport-layer wrapper with ports and reliability info.
IP packet — network-layer wrapper with source and destination IP addresses.
Ethernet frame — data-link wrapper with source and destination MAC addresses on a LAN.
Bits — the actual 1s and 0s on the cable or radio.

Lesson 1 only previews this stack. Lesson 2 traces encapsulation hop by hop and puts a layer number on each of these names. For now, remember: each layer adds its own header, and the PDU name changes with the layer.

Why hubs produced collisions

Early Ethernet often used a hub. A hub repeats incoming electrical signals out every other port — it does not make forwarding decisions. Every device attached to the hub shares one collision domain.

Collision domain vs broadcast domain

A collision domain is the set of devices whose transmissions can interfere with each other — if two of them send at the same instant, the signals collide. A broadcast domain is the set of devices that receive each other's broadcast messages, the ones addressed to everybody at once. The distinction matters because switches and routers treat them differently: every switch port is its own collision domain, but all of a switch's ports remain in one broadcast domain. It takes a router to separate broadcast domains.

Half-duplex Ethernet could not send and receive at the same time on one port.
CSMA/CD — Carrier Sense Multiple Access with Collision Detection — let devices listen before transmitting and back off if a collision occurred.
A switch creates a separate collision domain per switch port. Traffic is forwarded only to the port that needs it.
Modern switched Ethernet normally runs full duplex, so collisions and CSMA/CD are largely historical — but the terms still appear on the CCNA exam.

Exam vocabulary

Spell it CSMA/CD, not “CDSMA.” Know that hubs share collision domains; switch ports isolate them; only a router breaks up a broadcast domain.

What you should retain

Networks connect endpoints using shared rules; LANs are local, WANs span distance.
Clients request; servers respond; roles can change per application.
Switches forward inside a LAN; routers connect networks; firewalls enforce policy.
The default gateway is where a device sends anything not on its own local network.
PDU names data at each layer — preview the stack before deep dives.
Hubs share collision domains; switches break them per port; only routers break broadcast domains.

Pause and predictNot scored — nothing is recorded

Before you read on

A laptop and a printer sit on the same office network and exchange a file. One device forwards it; one device on the network is not involved at all. Which is which — and why?

Apply

Trace the Connection

Use the small office topology below. Answer each step before moving on. This is practice, not a graded exam.

Small office topology

Laptop-A → AP-1 → SW-1 → FW-1 / RTR-1 → Internet

Web-Srv → SW-1 (same LAN)

Step 1 of 5

Which device is the client in this scenario? A user on Laptop-A opens a web page from Web-Srv.

Section quiz follows Lesson 2. It covers Lessons 1 and 2 together, and it is tracked as its own Week 1 milestone — you can finish this lesson without it.

Lab preparation

Prepare for the lab

Assigned later this weekWeek 1

Week 1 Lab 1 — ARP & ICMP on the LAN

Later this week you will use Packet Tracer Simulation mode to watch ARP discover a local host, then follow ICMP Echo Request and Reply across a switch. It unlocks after Lesson 4, which teaches every protocol it asks you to identify.

This lesson prepares you to:

• Device roles on a simple LAN topology
• Local vs remote forwarding decisions
• Collision and broadcast domains in practice

Opens after Lesson 4 and the Local Delivery quiz. The lab walkthrough is subscriber content; its overview and objectives are open to everyone.

Preview lab overview

Study deeper

Topic guides extend this lesson — they do not replace the first-party walkthrough above.

Network Devices & Cabling

For the full device-role reference and the physical topology diagrams behind the seven devices above

Open guide

OSI Model & Packet Flow

For the complete PDU stack and encapsulation detail — Lesson 2 teaches this properly; open this if you want the full field reference now