This first lesson kicks off your CCNA journey by introducing the fundamental pieces of a network.
It explains what a network is, describes the difference between clients and servers, and gives a
first look at switches, routers and firewalls. No prior networking experience is required—only
basic familiarity with computers.
What is a network?
A computer network is a digital communications system that allows devices (also called nodes)
to share resources and exchange data. Even a simple setup of two PCs connected by a cable forms a
network capable of sending files back and forth. In larger networks, different types of devices
work together to enable communication between many hosts and across long distances.
Core Concepts
Clients
Devices (PCs, laptops, smartphones) that initiate connections and request services.
Servers
Systems or applications that provide functions to clients—web, file, DNS, DHCP, and more.
Switches
Layer‑2 devices that connect hosts within a LAN and forward frames using MAC addresses.
Routers
Layer‑3 devices that forward packets between networks and route traffic to the Internet.
Firewalls
Security appliances that monitor and filter network traffic based on configured rules.
Device Overview
Clients (end hosts)
Function: Initiate communication by requesting data or services from other systems.
Behaviour: Use temporary source ports to talk to servers on well‑known ports (such as 80/443 for web).
Dual roles: A device can act as a client and a server at the same time—for example, a smartphone requesting a webpage (client) and sharing a file via AirDrop (server).
Servers
Function: Provide services, content or processing power to clients.
Common services: Web (HTTP/HTTPS), file sharing (SMB/NFS), DNS, DHCP, authentication services like RADIUS.
Implementation: Can run on dedicated hardware, virtual machines or cloud instances. Any host can be configured to act as a server for a particular service.
Resilience: Important servers often use redundancy and backups to maintain availability.
Switches
Purpose: Connect multiple devices within a Local Area Network (LAN) and forward traffic based on MAC addresses.
Ports: Provide many interfaces—typically 24 or more—for connecting PCs, servers and printers.
Forwarding: Build a table mapping MAC addresses to ports so that frames are sent only where needed.
Scope: Operate within a single LAN and do not connect different networks or route traffic to the Internet.
Routers
Purpose: Forward packets between networks—also known as internetworking. Routers allow communication between different LANs and provide access to the Internet.
Interfaces: Have fewer ports than switches; each interface typically connects to a different network or subnet.
Routing decisions: Use IP addresses and routing tables to choose the best next hop for each packet.
Additional roles: Provide the default gateway for hosts, perform Network Address Translation (NAT) for Internet access and, on integrated devices, offer basic security features.
Firewalls
Role: Protect the network by inspecting and controlling traffic entering and leaving according to security policies.
Deployment: Can sit at the network edge (between internal networks and the Internet) or between different internal zones.
Types: Network firewalls are dedicated appliances; host‑based firewalls are software running on individual computers.
Advanced features: Next‑generation firewalls provide deeper inspection, intrusion prevention, URL filtering and malware detection.