Guedi Training Institute

04/08/2026

03/08/2026

IPv4 Datagram Header
Introduction to IPv4

Internet Protocol Version 4 (IPv4) is the fourth version of the Internet Protocol and the most widely used protocol for communication across computer networks. Introduced in 1983 as part of the ARPANET project, IPv4 provides logical addressing and packet routing, allowing devices to communicate over local networks and the Internet.

An IPv4 address consists of 32 bits (4 bytes) and is written in dotted-decimal notation, where four decimal numbers (octets) are separated by periods. For example:

192.168.1.1

Every IPv4 packet contains a header followed by the data (payload). The header stores important control information that enables routers and destination devices to deliver packets correctly and efficiently.
IPv4 Packet Structure

An IPv4 packet is divided into two main parts:

Header – Contains control information required for routing and delivery.
Payload (Data) – Contains the actual information being transmitted.

The IPv4 header has a minimum size of 20 bytes and can increase to 60 bytes if optional fields are included.
Key Features of IPv4

IPv4 provides several important networking capabilities:

Uses 32-bit addressing, allowing approximately 4.29 billion unique addresses (2³²).
Operates as a connectionless protocol, meaning each packet is routed independently.
Supports communication across LANs, WANs, and the Internet.
Provides logical addressing for network devices.
Supports both static and dynamic IP address assignment (such as through DHCP).
Supports Unicast, Broadcast, and Multicast communication.
Allows packet fragmentation when packets exceed the Maximum Transmission Unit (MTU).
Supports subnetting using Variable Length Subnet Masking (VLSM).
Uses the Address Resolution Protocol (ARP) to map IPv4 addresses to physical (MAC) addresses.

IPv4 Address Classes

Traditionally, IPv4 addresses were divided into five classes:

ClassPurposeClass ALarge networks with many hostsClass BMedium-sized networksClass CSmall networksClass DMulticast communicationClass EExperimental and research purposes

Although modern networks primarily use Classless Inter-Domain Routing (CIDR), understanding address classes remains useful for learning networking fundamentals.
Characteristics of IPv4

IPv4 has the following characteristics:

Uses 32-bit numerical addresses.
Addresses are written in dotted-decimal format.
The standard header contains 12 main fields.
The header size ranges from 20 to 60 bytes.
Supports Unicast, Broadcast, and Multicast communication.
Allows routers to fragment packets when necessary.
Supports subnetting through VLSM.
Uses ARP for resolving IP addresses to MAC addresses.
Can be configured manually (Static IP) or automatically using DHCP.
Compatible with many routing protocols, including RIP, OSPF, and BGP.

IPv4 Datagram Header Fields

The IPv4 header contains several fields that help deliver packets accurately across a network.

Header FieldSizeDescriptionVersion4 bitsIdentifies the IP version. The value is always 4 for IPv4.Header Length (IHL)4 bitsSpecifies the size of the IPv4 header in 32-bit words. Minimum value is 5 (20 bytes) and maximum is 15 (60 bytes).Type of Service (ToS) / DSCP8 bitsIndicates packet priority and quality of service requirements, such as low delay or high reliability.Total Length16 bitsSpecifies the total size of the packet, including both the header and payload. Maximum packet size is 65,535 bytes.Identification16 bitsUniquely identifies a packet so fragmented pieces can be reassembled correctly.Flags3 bitsControls fragmentation. Includes Reserved, Don't Fragment (DF), and More Fragments (MF) flags.Fragment Offset13 bitsIndicates the position of each fragment within the original packet.Time to Live (TTL)8 bitsLimits how many routers (hops) a packet can pass through before being discarded, preventing routing loops.Protocol8 bitsIdentifies the transport protocol carried in the payload, such as TCP (6), UDP (17), or ICMP (1).Header Checksum16 bitsDetects errors in the IPv4 header during transmission.Source IP Address32 bitsThe IPv4 address of the sending device.Destination IP Address32 bitsThe IPv4 address of the receiving device.Options (Optional)VariableStores optional information used for diagnostics, testing, routing, or network management.PaddingVariableAdds extra bits so the header length remains a multiple of 32 bits.
How an IPv4 Datagram Is Processed

When a device sends data over a network, IPv4 performs the following steps:

Creates an IPv4 packet by adding a header to the data.
Assigns the source and destination IP addresses.
Calculates the header checksum.
Sets the TTL value to prevent endless routing loops.
Sends the packet to the next router.
Each router decreases the TTL by one and forwards the packet toward its destination.
If the packet is larger than the network's MTU, it may be fragmented and later reassembled by the receiving device.

Why the IPv4 Header Is Important

The IPv4 header contains all the information needed for successful packet delivery. Network devices use the header to:

Identify the sender and receiver.
Determine the best routing path.
Detect transmission errors.
Control packet fragmentation.
Prevent packets from circulating indefinitely.
Identify the transport-layer protocol carrying the data.

Without the IPv4 header, routers and computers would be unable to deliver data reliably across networks.
Key Takeaway

The IPv4 Datagram Header is the control section of every IPv4 packet. It contains essential information such as IP addresses, packet size, routing details, fragmentation information, protocol type, and error-checking data. Understanding each header field is fundamental for learning networking, troubleshooting communication problems, and analyzing network traffic in cybersecurity.

02/08/2026

-8
What is Internet Protocol Version 4 (Pv4) ?

Unlike the speed of communication on the internet, we all know physical mail is slow. However, one thing remains the same: you need an address to send information. A person mails a note, message, or, hopefully, some cool Bunny merch to a recipient. The recipient then sees the address of the sender and can choose whether to open the letter or package or even respond to the sender’s address with a message of their own.

The difference lies in what the address looks like. Instead of numbers and words like a home address uses, internet addresses use numbers defined by the Internet Protocol (IP). That, and the fact that what you send is chopped up and sent in multiple packages, often called packets, through a process called fragmentation. It’s a lot like someone mailed your Bunny merch one sleeve at a time, and you have to sew it back together.

A final piece to our analogy is the network routers, which are like mail sorters at different post offices worldwide to ensure packages (packets) are routed ever closer to their destination.

IP version 4 (IPv4) was the first major Internet Protocol version defined in RFC 791 in 1981 and first deployed in 1982. It is still the dominant Internet Protocol. Let’s take a closer look at how IPv4 addressing works.
What is IPv4 addressing

IPv4 addresses are generally presented in dot-decimal notation, which consists of four octets (four groups of eight bits) of numbers separated by periods; for example, 198.51.100.1.

Each address is split into two parts: the network and the host. The network tells the internet which hosts are related. This distinction speeds up routing decisions since routers can just check which network the traffic belongs to instead of knowing the location of billions of individual IPs. It’s like getting off an elevator at a hotel and seeing a sign that shows rooms 200 to 214 are down the hallway to your left, and rooms 215 to 230 are to the right. A router just needs to know where the related groups of networks are.

For example, in the address 198.51.100.1, 198.51.100 is the network identifier, and the last octet .1 is the host identifier. In this case, the network identifier takes the first 24 bits and makes them static, and the host identifier takes the remaining 8 bits, where each possible value represents a single host. The network identifier can also be represented in slash notation: 198.51.100.1/24.

We usually reference the hosts on the internet through domain names or URLs rather than IP addresses. Domain names are translated to IP addresses and vice versa during DNS resolution, which is performed by the Domain Name System (DNS).
First and last IPv4 subnet addresses

The subnetwork and the broadcast address should not be used by any host. The first address in a subnetwork is reserved and used to identify the subnet. This is the address where all host bits are 0. The last address in a subnet is also reserved and used as a broadcast address. This is the address where all host bits are 1. For the example network 198.51.100.0/24, we have the following subnetwork and broadcast addresses:

Type Binary form Dot-decimal notation

Subnetwork address 11000110.00110011.01100100.00000000 198.51.100.0/24 Broadcast address 11000110.00110011.01100100.11111111 198.51.100.255/24

01/08/2026

What Is Ethernet?

Ethernet is the standard technology (IEEE 802.3) that lets devices on a wired network communicate without their data colliding. It defines how devices are addressed, how fast they can transmit, what cables are used, and what happens if two devices try to send data at the same time.
What Is an Ethernet Switch?

A switch is a networking device that connects multiple wired devices (computers, printers, servers, access points) within a Local Area Network (LAN). Unlike a router — which connects separate networks together (e.g., your LAN to the internet) — a switch's job is to manage traffic between devices on the same network.

Switch vs. Hub: A hub blindly broadcasts data to every device. A switch is smarter — it can send extra bandwidth to specific ports without slowing down the rest of the network.
How a Switch Works (step by step)

Every network device has a unique hardware ID called a MAC address.
When a device sends data, the switch reads the MAC address and notes which port that device is connected to, storing it in a MAC address table.
For each incoming packet, the switch checks its table: If it knows the destination's port → sends the data directly there.
If not → broadcasts to all ports until it gets a reply, then updates its table.

Switch Categories by Network Size

Access switches – connect end devices at the edge
Distribution switches – aggregate traffic from access switches
Core switches – handle the highest-speed backbone traffic

Main Internal Components

CPU – manages overall switch operations
Forwarding engine – decides where each packet goes using lookup tables
Replication engine – duplicates packets when sending to multiple destinations
Switch fabric – the internal hardware/software pathway connecting ports
Queues/buffers – temporarily hold data to prevent overload

Types of Switches

Type Description

Managed Configurable, supports security rules/authentication, used in businesses

Unmanaged Plug-and-play, no configuration, basic connectivity only
Key Features

Operates at Layer 2 (Data Link Layer) of the OSI model
Uses MAC addresses to direct traffic
Supports full-duplex (send + receive simultaneously)
Supports unicast (1-to-1) and multicast (1-to-many)

Advantages / Disadvantages

Pros: reduces downtime, improves bandwidth use, better security options (managed switches), lowers load per device.

Cons: higher cost, a switch failure can disrupt the network, multicast traffic needs careful setup, troubleshooting can be harder than with simple hubs.

31/07/2026

What is Command Line Interface (CLI)?

CLI stands for:

Command Line Interface
Command Line Interpreter
Command Line Input

Command Line Input

CLI is a command line program that accepts text input to execute operating system functions.

In the 1960s, using only computer terminals, this was the only way to interact with computers.

In the 1970s an 1980s, command line input was commonly used by Unix systems and PC systems like MS-DOS and Apple DOS. Today, with graphical user interfaces (GUI), most users never use command-line interfaces (CLI).However, CLI is still used by software developers and system administrators to configure computers, install software, and access features that are not available in the graphical interface.
Examples

The software package manager npm uses command line input to install software:
Windows Example

C:\>npm install mysoftware
Mac OS Example

>npm install mysoftware

You can navigate your folders (directories) with command line commands:
Windows Example

C:\Users\myuser>cd .. C:\Users\>cd .. C:\>

Use CLI commands with great attention!!!

Wrong use can easily delete files or destroy your computer system completely.
Basic Linux CLI Commands

Command Description

ls List the directory (folder) system.

cd pathnameChange directory (folder) in the file system.

cd .. Move one level up (one folder) in the file system.

cp Copy a file to another folder.

mv Move a file to another folder.

mkdir Creates a new directory (folder).

rmdir Remove a directory (folder).

clear Clears the CLI window.

exit Closes the CLI window.

man commandShows the manual for a given command.

Basic Windows CLI Commands

Command Description

dir List the directory (folder) system.

cd pathnameChange directory (folder) in the file system.

cd \ Move to the root folder of the file system.

cd .. Move one level up (one folder) in the file system.

copy Copy a file to another folder.

move Move a file to another folder.

type filenameType a file.

mkdir or md Creates a new directory (folder).

rmdir or rd Removes a directory (folder).

cls Clears the CLI window.

exit Closes the CLI window.

help commandShows the manual for a given command.

DOS Commands Help

You can display all available commands with the help command:
Example

C:\Users\myuser>help
Command Help

For more information on a specific command, type help + command-name
Example

C:\Users\myuser>help date

GitHub CLI offers a powerful CLI tool that brings its core functionality directly into your terminal. With GitHub CLI, you can clone repositories, create and review pull requests, check the status of issues, and even trigger GitHub Actions from a single, scriptable interface.

30/07/2026

Understanding the OSI Model – The Foundation of Networking!

Have you ever wondered how your computer communicates with a website, email server, or another device?

The OSI (Open Systems Interconnection) Model provides a simple way to understand how data travels across a network. It divides communication into 7 layers, where each layer has a specific job to ensure data is transmitted accurately and securely.

The 7 Layers of the OSI Model

🔹 Layer 7 – Application: Network services for applications (Web, Email, FTP, DNS)

🔹 Layer 6 – Presentation: Data translation, encryption, and compression

🔹 Layer 5 – Session: Establishes and manages communication sessions

🔹 Layer 4 – Transport: Reliable or fast data delivery using TCP or UDP

🔹 Layer 3 – Network: Uses IP addresses to route data

🔹 Layer 2 – Data Link: Uses MAC addresses and detects transmission errors

🔹 Layer 1 – Physical: Transmits data through Ethernet cables, fiber optics, or Wi-Fi

Why is the OSI Model Important?

Makes networking easier to understand
Helps troubleshoot network problems
Provides a common standard for network communication
Essential knowledge for CCNA, Cybersecurity, Cloud Computing, and Network Engineering professionals

At Guedi Training Institute (GTI), our goal is to make complex IT concepts simple through practical training and visual learning resources.

What networking topic would you like us to explain next? Let us know in the comments!

29/07/2026

TCP/IP Model – Short Summary
The TCP/IP (Transmission Control Protocol/Internet Protocol) model is the standard communication model that allows devices to exchange data over networks and the Internet. It breaks data into packets, assigns destination addresses, transmits the packets through the network, and reassembles them at the destination to ensure reliable communication.
The 4 Layers of TCP/IP
Application Layer – Creates and receives data from applications such as web browsers, email, and file transfer.
Transport Layer – Uses TCP or UDP to divide data into segments, manage communication, and ensure reliable or fast delivery.
Internet Layer – Uses IP addresses to route packets across different networks to the correct destination.
Network Access Layer – Sends and receives data over physical media such as Ethernet cables or Wi-Fi.
How TCP/IP Works
An application creates data.
TCP or UDP prepares the data for transmission.
IP adds the source and destination addresses.
The data travels through routers and networks.
The destination device receives, reassembles, and delivers the original data to the application.
Key Points
TCP provides reliable, error-checked communication.
IP is responsible for addressing and routing packets.
UDP is faster but does not guarantee delivery.
TCP/IP is the foundation of the Internet and modern computer networks.
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28/07/2026

Day3
Ever wondered how your laptop actually talks to the internet?
It's not magic — it's a chain of devices working together, each with one job:
Modem → converts the ISP signal into something your network understands
Router → assigns IP addresses and connects your network to the internet
Switch → connects all your wired devices (PCs, printers, cameras)
WAP (Wireless Access Point) → gives your laptops and phones Wi-Fi
Firewall → filters traffic and blocks unauthorized access
Server → delivers files, web pages, email, and more
The flow looks like this:
Internet → Modem → Router → Switch → WAP → Firewall → Server
Every device in this chain has a single responsibility. Understanding this flow is the foundation of networking — whether you're setting up a home office or studying for a certification like CompTIA Network+ or CCNA.
What part of networking do you wish you understood better when you started?
Cisco Networking Academy The ITInfrastructure Report Cybersecurity TechEducation

27/07/2026

Day2
What Are Network Interfaces?

A network interface is a physical or virtual connection point that allows a computer or device to talk to a network. It acts as a bridge between your device's internal software and the outside data line or radio waves.

The Cable Connection Types
When working with network interface cabling, you'll encounter several main categories that each serve different purposes in your network infrastructure.

Copper Cabling
Copper cables are the workhorses of local networking. The most common

type is twisted pair cable, which comes in two varieties:
Unshielded Twisted Pair (UTP) - Standard Ethernet cables you see everywhere
Shielded Twisted Pair (STP) - Protected against electromagnetic interference

These cables use RJ-45 connectors and support various speeds depending on the category. Cat5e supports up to 1 Gbps, while Cat6 can handle 10 Gbps over shorter distances.

Fiber Optic Cabling
Fiber-optic cables transmit data using light rather than electrical signals. This makes them ideal for long distances and high-speed connections. You'll encounter two main types:
Single-mode fiber - Long distance, high bandwidth
Multi-mode fiber - Shorter distances, cost-effective for building connections

Interface Speed and Duplex
Every network interface operates at a specific speed and duplex setting. Speed refers to how fast data can travel (10 Mbps, 100 Mbps, 1 Gbps, etc.), while duplex determines whether the interface can send and receive simultaneously:
Half-duplex - Send OR receive, but not both at once
Full-duplex - Send AND receive simultaneously

Choosing the Right Interface and Cable
When planning your network infrastructure, consider these factors:
Distance - Copper works well up to 100 meters, fiber extends much further
Speed requirements - Match your interface capabilities to your bandwidth needs
Environment - Industrial settings might require shielded cables
Budget - Copper is generally less expensive than fiber
Understanding these network fundamentals helps you make informed decisions about your network design and troubleshooting approach.

24/07/2026

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