Network Fundamentals

Introduction to Network Fundamentals

Computer networking is one of the most important foundations of cyber security.
Almost every modern computer, smartphone, server, website, cloud service,
banking system, and business application depends on a network to communicate.

Before a security professional can understand network attacks, firewalls,
intrusion detection, packet analysis, or network monitoring, they must first
understand how networks work.

A computer network allows two or more computing devices to communicate and
exchange information using agreed communication rules called protocols.

What Is a Computer Network?

A computer network is a collection of connected devices that
can communicate with one another and share data, services, applications, or
resources.

The devices connected to a network may include:

• Desktop computers
• Laptops
• Smartphones
• Servers
• Printers
• Network-attached storage devices
• Routers
• Switches
• Wireless access points
• Security appliances
• Internet of Things devices

Why Do We Need Networks?

Networks allow people and systems to communicate and share resources.
For example, an organization may use a network to allow employees to access
shared files, printers, databases, applications, email systems, and the
internet.

Without networking, modern services such as cloud computing, online banking,
video conferencing, social media, online learning, and web applications would
not operate in the same way.

Basic Network Components

1. Hosts

A host is a device that participates in network communication.
A computer, phone, server, or other network-connected device can function as
a host.

2. Clients

A client is a device or application that requests a service
from another system.

For example, when a web browser requests a webpage from a web server, the
browser acts as a client.

3. Servers

A server provides services or resources to other devices.
Examples include:

• Web servers
• Database servers
• File servers
• Mail servers
• DNS servers
• Authentication servers

4. Network Interface Card

A Network Interface Card (NIC) allows a device to connect to
a network. A NIC may provide a wired Ethernet connection or wireless
connectivity.

5. Switches

A switch connects devices within a local network and forwards network traffic
to the appropriate destination.

6. Routers

A router connects different networks and determines where network traffic
should be forwarded.

7. Wireless Access Points

A wireless access point provides wireless connectivity so devices such as
laptops and smartphones can communicate with a network.

8. Firewalls

A firewall controls network traffic according to security rules. It can allow,
deny, or inspect traffic based on defined policies.

Network Communication

When one device communicates with another, information is broken into smaller
units called packets.

Packets contain information required for delivering data from a source to a
destination. Network devices and protocols work together to move these packets
through the network.

For example, when you open a website:

1. Your device creates a request.
2. The request is divided into network data.
3. The data is transmitted through the network.
4. Routers and other network devices help forward the traffic.
5. The destination server receives the request.
6. The server sends a response.
7. Your device receives and processes the response.

Network Types

LAN — Local Area Network

A Local Area Network (LAN) connects devices within a limited
geographical area such as an office, classroom, laboratory, home, or building.

WAN — Wide Area Network

A Wide Area Network (WAN) connects networks across larger
geographical areas.

The internet is the world’s most well-known example of a large interconnected
network.

WLAN — Wireless Local Area Network

A Wireless Local Area Network provides LAN connectivity
through wireless technologies such as Wi-Fi.

Network Topology

Network topology describes how devices and connections are arranged.

Common network topologies include:

• Star topology
• Bus topology
• Ring topology
• Mesh topology
• Hybrid topology

Star Topology

In a star topology, devices connect to a central device, commonly a switch.
This design is widely used in modern Ethernet networks.

Mesh Topology

In a mesh topology, devices have multiple network paths between them.
This can provide redundancy and resilience.

Important Networking Terms

Bandwidth

Bandwidth refers to the capacity of a network connection to
transfer data over a period of time.

Latency

Latency is the amount of time required for data to travel
from one point to another.

Packet

A packet is a unit of data transmitted across a network.

Protocol

A protocol is a set of rules that defines how devices
communicate and exchange information.

Node

A node is any device or connection point participating in a
network.

Networking and Cyber Security

Networking knowledge is essential for cyber security because many attacks
involve network communication.

Security professionals need to understand networking in order to analyze:

• Suspicious network traffic
• Unauthorized connections
• Malware communication
• Port scanning
• Denial-of-service attacks
• Credential attacks
• Data exfiltration
• Man-in-the-middle attacks
• Firewall events
• Intrusion detection alerts

For example, if a security analyst sees a computer making repeated outbound
connections to an unusual external server, understanding network addresses,
ports, protocols, and traffic patterns helps the analyst investigate the event.

Simple Network Example

Consider a small Dwallnet training laboratory containing:

• 10 student computers
• 1 instructor computer
• 1 network printer
• 1 server
• 1 network switch
• 1 router
• 1 firewall
• Internet connectivity

The computers can connect to the switch. The switch provides local
communication between devices, while the router provides connectivity between
the local network and other networks. The firewall applies security rules to
network traffic.

Network Communication Flow

Student Computer
       |
       v
     Switch
       |
       v
    Firewall
       |
       v
     Router
       |
       v
    Internet
       |
       v
   Web Server

This simple model demonstrates how several networking technologies can work
together to provide communication while security controls monitor and restrict
traffic.

Network Fundamentals and Security

A cyber security professional should be able to answer basic questions such
as:

• What device generated this traffic?
• Where is the traffic going?
• Which protocol is being used?
• Which port is being used?
• Is the destination internal or external?
• Is the communication expected?
• Which network device forwarded the traffic?
• Was the traffic allowed or blocked?

These questions form the foundation for network security monitoring and
troubleshooting.

Practical Cyber Security Exercise

Imagine that a computer in a training laboratory suddenly begins communicating
with an unknown external server.

As a beginner security analyst, identify the information you would want to
collect:

1. Source device
2. Source IP address
3. Destination IP address
4. Destination port
5. Protocol
6. Time of communication
7. Amount of data transferred
8. Firewall decision
9. Relevant security logs

Do not attempt to attack or disrupt the external system. The objective is to
practice identifying and analyzing network information responsibly.

Code Example: Basic Network Information with PHP

<?php

echo "Server IP: " . $_SERVER['SERVER_ADDR'] . PHP_EOL;
echo "Client IP: " . $_SERVER['REMOTE_ADDR'] . PHP_EOL;
echo "Server Port: " . $_SERVER['SERVER_PORT'] . PHP_EOL;
echo "Request Method: " . $_SERVER['REQUEST_METHOD'] . PHP_EOL;

?>

This example demonstrates how a PHP application can access basic server and
request information. In a production security system, such information should
be handled carefully and should not be exposed unnecessarily to users.

Key Security Principle

Understanding how normal network communication works is necessary before
trying to identify abnormal or malicious communication.

A security professional should first understand the baseline: what devices
exist, how they normally communicate, which services are expected, and which
network paths are legitimate.

Lesson Summary

Network fundamentals provide the foundation for understanding computer
communication and cyber security. A network connects devices so that they can
exchange information and share resources.

Important concepts introduced in this lesson include hosts, clients, servers,
NICs, switches, routers, firewalls, packets, protocols, LANs, WANs, bandwidth,
latency, and network topology.

The following lessons will build on these concepts by examining LAN and WAN
networks, IP addresses, MAC addresses, ports, protocols, TCP/IP, DNS, DHCP,
routers, switches, firewalls, and basic network troubleshooting.