What Is a Network?
A computer network is a digital telecommunications system that allows multiple devices called nodes to communicate with each other and share resources such as data, applications, printers, and internet access.
In simple terms, a network enables devices to talk to each other.
Clients and Servers
Client
A client is a device that requests or accesses services provided by another device.
Examples include:
A laptop accessing a website
A mobile phone connecting to email services
Server
A server is a device that provides services or resources to clients.
Examples include:
Web servers hosting websites
File servers storing shared data
Email servers managing mail communication
Important Note:
The same device can act as both a client and a server, depending on the situation.
For example, in a peer-to-peer (P2P) network, devices share resources directly without a dedicated central server.
Network Devices and Their Roles
Switches (Layer 2 Devices)
Switches operate at Layer 2 (Data Link Layer) of the OSI model and are mainly used within a local network.
Key functions:
Provide connectivity between devices within the same LAN (Local Area Network)
Have multiple ports to connect end devices such as PCs, printers, and servers
Forward data based on MAC addresses
Routers (Layer 3 Devices)
Routers operate at Layer 3 (Network Layer) of the OSI model and are responsible for directing traffic between networks.
Key functions:
Connect multiple LANs together
Enable communication between local networks and the internet
Route data using IP addresses
Routers typically have fewer interfaces than switches but play a critical role in network connectivity.
Firewalls (Layer 3, 4, and 7)
A firewall is a specialized network security device designed to monitor and control traffic entering or leaving a network.
Key functions:
Enforce security policies using predefined rules
Filter traffic based on IP addresses, ports, protocols, or applications
Protect networks from unauthorized access and cyber threats
Firewalls can be placed:
Outside the network (between the internet and internal network)
Inside the network (to protect sensitive segments)
Next-Generation Firewalls (NGFW)
Firewalls are referred to as Next-Generation Firewalls (NGFWs) when they include advanced security features such as:
Application-level inspection (Layer 7)
Intrusion Prevention Systems (IPS)
Deep packet inspection
Threat intelligence and malware protection
Host-Based Firewalls
A host-based firewall is a software firewall installed directly on a device such as a laptop or server.
Examples:
Windows Defender Firewall
Linux iptables / nftables
These firewalls:
Filter inbound and outbound traffic at the host level
Provide an additional security layer even inside trusted networks