🧠 INTRODUCTION — Why We Even Talk About OSI and TCP/IP
Imagine two computers — one in the UK and one in India — trying to talk to each other over the internet.
They speak different languages, have different hardware, different operating systems, and yet, they can exchange data perfectly.
💡 How? Because both follow a set of standard communication rules, known as networking models — mainly the OSI model and the TCP/IP model.
These models are like “blueprints” or “recipes” for how data moves from one computer to another — layer by layer.
🌍 WHY WE NEED MODELS
Without these models:
- Devices from different manufacturers couldn’t talk to each other.
- Software developers wouldn’t know how to send or receive data.
- Networks would be chaotic, like trying to build roads without traffic rules.
✅ The models bring order, structure, and compatibility.
🏗️ THE TWO MODELS
| Model | Full Name | Layers | Purpose |
|---|---|---|---|
| OSI Model | Open Systems Interconnection | 7 Layers | A conceptual model used for understanding and teaching networking. |
| TCP/IP Model | Transmission Control Protocol / Internet Protocol | 4 (or 5) Layers | A practical model that the real Internet actually uses. |
🔹 OSI MODEL (7 Layers) — “All People Seem To Need Data Processing”
| No. | Layer Name | Function | Example Devices / Technologies |
|---|---|---|---|
| 7 | Application | Where users interact with network applications | Web browsers (Chrome, Edge), Email (Outlook), FTP, HTTP, DNS |
| 6 | Presentation | Converts data formats — encryption, compression | JPEG, MP3, SSL/TLS encryption |
| 5 | Session | Manages sessions (start, maintain, end communication) | Logins, Remote sessions (RDP), NetBIOS |
| 4 | Transport | Breaks data into segments, ensures reliable delivery | TCP (reliable), UDP (fast) |
| 3 | Network | Decides path for data, handles IP addressing | Routers, IP, ICMP |
| 2 | Data Link | Creates frames, handles MAC addresses | Switches, Ethernet, ARP, VLANs |
| 1 | Physical | Sends bits over cables or air | Cables, Wi-Fi signals, Hubs, Repeaters |
💬 Example:
When you send a WhatsApp message:
- App layer → WhatsApp
- Transport → TCP ensures message delivered
- Network → IP finds the route
- Data Link → Switch sends frame to next device
- Physical → Electrical/wireless signals transmit bits
🔹 TCP/IP MODEL (4 Layers)
| Layer | Function | Example Protocols / Devices |
|---|---|---|
| Application | User interaction, data formatting | HTTP, DNS, SMTP |
| Transport | Reliable delivery | TCP, UDP |
| Internet | Logical addressing and routing | IP, ICMP |
| Network Access | Physical transmission | Ethernet, Wi-Fi, Switches, Cables |
🧩 Notice how TCP/IP combines the 7 OSI layers into 4 practical groups.
⚖️ DIFFERENCE BETWEEN OSI AND TCP/IP
| Feature | OSI Model | TCP/IP Model |
|---|---|---|
| Type | Theoretical / Reference | Practical / Implemented |
| Layers | 7 | 4 (or 5) |
| Developed by | ISO (International Organization for Standardization) | DoD (US Department of Defense) |
| Usage | Teaching and design concept | Real-world Internet communication |
| Example | Explains “how data should move” | Explains “how data actually moves” |
🌐 WHY IT’S IMPORTANT TO THE IT WORLD
OSI and TCP/IP models are not just for network engineers — they are the foundation of all digital communication:
- Web developers use them (HTTP, HTTPS).
- System admins use them (DNS, DHCP, IP addressing).
- Cybersecurity experts rely on them (firewall rules, intrusion detection).
- Cloud engineers use them (virtual networks, subnets, load balancers).
- Even programmers depend on them (sockets, ports).
🖥️ Basically — every IT system that communicates (from IoT devices to cloud servers) follows these rules.
🧩 SIMPLE REAL-WORLD EXAMPLE
Scenario: You open www.google.com in your browser.
- Application Layer: Browser sends request → HTTP GET
- Transport Layer: TCP ensures your data reaches Google safely
- Network Layer: IP finds Google’s server address
- Data Link Layer: Ethernet frame moves through switch
- Physical Layer: Data travels as electrical signals or Wi-Fi waves
When Google replies, the process reverses layer by layer!
🚀 EASY MEMORY TIP
OSI (7 Layers):
👉 “All People Seem To Need Data Processing”
A – Application
P – Presentation
S – Session
T – Transport
N – Network
D – Data Link
P – Physical
TCP/IP (4 Layers):
👉 “All Tigers In Nepal”
A – Application
T – Transport
I – Internet
N – Network Access
💡 SIMPLE ANALOGY — “The Letter Analogy”
Imagine sending a letter:
| Step | OSI Layer | Explanation |
|---|---|---|
| You write the message | Application | User creates data |
| Translate to English | Presentation | Data formatting |
| Attach in envelope | Session | Communication setup |
| Hand it to postal service | Transport | Delivery control |
| Post office sorts mail | Network | Finds best route |
| Truck delivers to your area | Data Link | Local delivery |
| Postman drops at door | Physical | Actual delivery |
✅ SUMMARY
- OSI = Theory → helps understand network communication.
- TCP/IP = Practice → the real-world version of OSI.
- Every IT system — networking, web, cloud, or even mobile apps — relies on these models.
- Knowing them gives you the foundation for CCNA, Security+, Cloud, or any IT field.
⚡ Amazing & Lesser-Known Facts About the OSI Model
🧩 1. The OSI Model Is Not Used Exactly in Real Networks
Most students think OSI runs the internet — but it doesn’t!
➡️ The Internet runs on the TCP/IP model, not OSI.
The OSI model was created by ISO in the 1980s to standardize communication, but the U.S. Department of Defense had already made TCP/IP — which became the real-world standard.
💬 Think of OSI as a teacher — it explains how things should work.
Think of TCP/IP as the worker — it actually does the job.
🧠 2. Every Time You Send a Message — You’re Using All 7 Layers
Even when you send a simple WhatsApp “Hi”:
- App layer (WhatsApp)
- Transport (TCP)
- Network (IP)
- Data link (Wi-Fi)
- Physical (your phone antenna)
📡 So — you’re using the entire OSI concept hundreds of times per day without realising it.
⚙️ 3. Layers Talk to Each Other Vertically
Each layer doesn’t talk sideways —
They only talk to the layer above and below them.
So, Layer 3 (Network) doesn’t know what Layer 7 (Application) is doing directly.
They pass data step by step, like passing a parcel through 7 hands.
💡 That’s why troubleshooting networks often involves finding which “layer” the problem is on.
🔍 4. Most Network Engineers Troubleshoot Using the OSI Model
When something breaks, they don’t panic — they think:
“Which layer is broken?”
Example:
- Can’t see the Wi-Fi? → Layer 1 (Physical)
- Can connect but no IP address? → Layer 2/3 (Data Link/Network)
- Can browse IP but not website name? → Layer 7 (Application, DNS issue)
🧠 OSI is not just theory — it’s a real-world diagnostic map.
🌐 5. It’s Not Just Networking — It’s All of IT
People think OSI is only for networking — wrong!
Every system that transfers data follows these concepts:
- Cloud computing (virtual networks, API calls)
- Cybersecurity (firewalls, proxy servers)
- Software apps (HTTP, HTTPS, SSL/TLS)
- IoT and Smart Devices (data packets between sensors)
💡 So OSI is the universal language of IT communication.
🔒 6. Most Cyberattacks Target Specific Layers
Attackers often choose which OSI layer to attack:
- Layer 7: DDoS, malware, phishing, SQL injection
- Layer 3: IP spoofing
- Layer 2: MAC flooding or ARP poisoning
- Layer 1: Physical tampering or wiretapping
🎯 Cybersecurity experts use OSI to understand where an attack is happening.
⚙️ 7. Encapsulation & De-Encapsulation Is the Hidden Magic
When you send data, each OSI layer wraps it with extra info — like adding envelopes inside envelopes.
- The Application layer says “this is a message.”
- The Transport layer adds port numbers.
- The Network layer adds IP addresses.
- The Data Link layer adds MAC addresses.
📦 This process is called Encapsulation.
When data reaches the receiver, it’s unwrapped layer by layer — called De-encapsulation.
That’s how your “message” turns into “bits” and back again.
🧱 8. Layers Aren’t Equal in Power
Some layers are hardware-based (Physical, Data Link).
Some are software-based (Application, Transport).
That’s why network engineers often work on layers 1–3, while software and security people deal with 4–7.
🕰️ 9. The OSI Model Came Before Wi-Fi or the Internet
The model was designed before modern networking even existed.
Yet it perfectly described how future networks would work — that’s why it’s still used in IT education and certification like CCNA, Security+, and CompTIA Network+.
💬 It’s like the “DNA” of networking — written decades ago, but still running the digital world.
🔢 10. There’s an 8th Hidden Layer (Jokingly Called “Layer 8”)
Engineers joke that there’s one more layer above all:
Layer 8 = The User (Human Error 😂)
Because no matter how perfect the system is, a human can still unplug the cable, forget the password, or block the wrong IP address.
💡 BONUS: Mind-Blowing Example
When you stream YouTube:
- App: YouTube player
- Presentation: H.264 video codec
- Session: Keeps the video connected
- Transport: TCP controls buffering
- Network: IP routes data packets
- Data Link: Wi-Fi or Ethernet frames
- Physical: Your phone signal or cable
⚡ And all this happens in milliseconds, millions of times every second!
🌍 REAL-WORLD EXAMPLES OF EACH OSI LAYER
Layer 7 – Application 🧑💻 (The User’s World)
What happens: You use apps that rely on the network.
Protocols: HTTP, HTTPS, DNS, SMTP, FTP, POP3, SNMP
Devices/Software: Web browsers, Email clients, YouTube, WhatsApp, Netflix, Zoom
💡 Real-life examples
- Opening Instagram → uses HTTP + HTTPS
- Sending an email → SMTP (send), IMAP/POP3 (receive)
- Searching on Google → DNS + HTTPS
- Watching Netflix → App communicates through HTTPS
- Talking on Teams/Zoom → Application layer handles chat & video session
🧠 If you can click or tap it, it’s Layer 7.
Layer 6 – Presentation 🎨 (Translator & Formatter)
What happens: Data is translated, encrypted, or compressed so both sides understand it.
Protocols: SSL/TLS, JPEG, MP3, MPEG, ASCII, PNG
Devices/Software: Web browsers, media players, VPN encryptors
💡 Real-life examples
- A secure padlock 🔒 on a website → SSL/TLS encryption
- Watching an MP4 video → decoding/compression
- Listening to Spotify → MP3 or AAC codec
- Saving Word docs as PDF → format conversion
- Viewing emojis 😊 on any device → character encoding
🧠 This layer makes sure the data “looks right.”
Layer 5 – Session 🔁 (Conversation Manager)
What happens: Starts, maintains, and ends sessions between two devices.
Protocols: NetBIOS, RPC, PPTP, SIP
Devices/Software: Web servers, conference apps
💡 Real-life examples
- Logging into Facebook and staying logged in
- A Zoom meeting staying active even if video buffers briefly
- RDP session between your laptop and a school server
- VoIP call connection setup using SIP
🧠 It’s like keeping the call line open while you talk.
Layer 4 – Transport 🚚 (Reliable Delivery)
What happens: Breaks data into segments, adds port numbers, ensures reliable delivery.
Protocols: TCP (reliable), UDP (fast)
Devices: Firewalls, load balancers, routers (some)
💡 Real-life examples
- Browsing websites → TCP (so pages load fully)
- Watching a YouTube livestream or gaming → UDP (for low delay)
- WhatsApp calls → UDP
- Downloading a file → TCP ensures 100% data received
- Speedtest.net uses both TCP and UDP to measure throughput
🧠 TCP = Postman with signature, UDP = Throw the parcel quickly!
Layer 3 – Network 🌐 (Finding the Route)
What happens: Finds the best path for packets to travel.
Protocols: IP, ICMP, IPSec, OSPF, RIP
Devices: Routers, Layer 3 switches
💡 Real-life examples
- GPS / Google Maps choosing fastest route = similar logic
- Your router assigning IPs (192.168.x.x)
- Running a ping command → ICMP test
- VPN masking your IP address
- Sending email from UK to USA → routers choose fastest route
🧠 Layer 3 is the “post office network.”
Layer 2 – Data Link 🧱 (Local Delivery)
What happens: Moves frames inside the same local network using MAC addresses.
Protocols: Ethernet, ARP, PPP, VLAN
Devices: Switches, network cards (NICs)
💡 Real-life examples
- Your Wi-Fi access point connecting your phone to your router
- A switch sending data to the right classroom PC
- Your printer reachable because it’s on the same VLAN
- Bluetooth headphones talking directly to your phone
🧠 Layer 2 knows “who is who” inside the same house.
Layer 1 – Physical ⚡ (The Wires & Signals)
What happens: Sends 1s and 0s as electrical, light, or radio signals.
Technologies: Ethernet cables, Fibre optics, Wi-Fi, Bluetooth, 4G/5G
Devices: Hubs, repeaters, cables, antennas
💡 Real-life examples
- Wi-Fi signal bars 📶 → quality of Layer 1 connection
- Ethernet cable unplugged → no physical link
- Fibre-optic Internet sending data as light
- AirPods Bluetooth → radio waves transmitting bits
🧠 It’s the actual highway the data travels on.
🔹 TCP/IP MODEL — PRACTICAL EXAMPLES
| TCP/IP Layer | Real-World Examples | Similar OSI Layers |
|---|---|---|
| Application | Web browsers, WhatsApp, Zoom, Email, Netflix | OSI 7-5 |
| Transport | TCP (web), UDP (gaming), QUIC (YouTube) | OSI 4 |
| Internet | Routers, IP addressing, VPN, Ping | OSI 3 |
| Network Access | Wi-Fi, Ethernet, Switches, Cables | OSI 1-2 |
💡 Quick Everyday Mapping
| Real-Life Task | TCP/IP Layers Used |
|---|---|
| Streaming Netflix | App (HTTPS), Transport (TCP), Internet (IP), Network Access (Wi-Fi) |
| Playing Fortnite | App (Game Server), Transport (UDP), Internet (IP), Network Access (5G or Ethernet) |
| Emailing someone | App (SMTP/IMAP), Transport (TCP), Internet (IP), Network Access (Wi-Fi) |
| Browsing Google | App (HTTP), Transport (TCP), Internet (IP), Network Access (Ethernet) |
| Making a Zoom Call | App (SIP/Zoom), Transport (UDP/TCP), Internet (IP), Network Access (Wi-Fi) |
⚙️ ANALOGY FOR MEMORY
| OSI Layer | Real-World Analogy |
|---|---|
| 7 | You writing a message |
| 6 | Translating it to recipient’s language |
| 5 | Starting a phone call |
| 4 | Speaking clearly, ensuring message delivered |
| 3 | Choosing the route for delivery |
| 2 | Addressing the letter locally |
| 1 | Physically sending it by post or courier |
🚀 Summary for Students
| Layer | What It Does | Common Examples You Already Use |
|---|---|---|
| 7 | User interaction | YouTube, Instagram, Gmail |
| 6 | Format & encryption | SSL padlock, file compression |
| 5 | Sessions | Logging into Netflix |
| 4 | Delivery | TCP/UDP in games & web |
| 3 | IP routing | VPN, Ping, Router |
| 2 | MAC & local links | Switch, Wi-Fi |
| 1 | Signals | Fibre, Cable, Bluetooth |
🧠 1. The Reality
- TCP/IP = what computers use
- OSI = what humans use to understand and fix problems
✅ Think of it like this:
TCP/IP runs networking.
OSI explains networking.
When something breaks, we need a clear way to think through the layers — that’s where OSI becomes your troubleshooting map.
⚙️ 2. Why We Still Use the OSI Model for Diagnosis
Even though real traffic follows TCP/IP,
the OSI model gives a universal language for engineers.
💬 For example:
- “This looks like a Layer 3 issue” → means it’s a routing (IP) problem.
- “Check Layer 1” → means cables, ports, and signals.
- “Layer 7 timeout” → means application issues like a slow website or DNS failure.
Every IT technician, Cisco engineer, or support team in the world understands those phrases instantly — even if the real traffic is TCP/IP.
🧩 3. How OSI and TCP/IP Relate in Troubleshooting
Here’s how you actually use OSI to diagnose TCP/IP networks:
| OSI Layer | TCP/IP Equivalent | Real-World Tools / Checks |
|---|---|---|
| Layer 1 – Physical | Network Access | Check cables, Wi-Fi signal, NIC lights, link status |
| Layer 2 – Data Link | Network Access | Check MAC addresses, VLANs, switch ports, ARP tables |
| Layer 3 – Network | Internet | Ping, traceroute, IP addressing, routing tables |
| Layer 4 – Transport | Transport | netstat, telnet, port check, firewall rules |
| Layer 5-7 – Session / Presentation / Application | Application | Browser, DNS, HTTP errors, SSL issues, server logs |
So you diagnose using OSI structure,
but test actual protocols that belong to TCP/IP.
🧰 4. Example – Troubleshooting Step-by-Step
🧩 Scenario:
User says:
“I can’t access learntechfromzero.com”
You apply OSI layers like this:
| OSI Layer | What You Check | Example Tool |
|---|---|---|
| 1 – Physical | Is the cable plugged? Is Wi-Fi connected? | Check NIC lights |
| 2 – Data Link | Is the correct VLAN or SSID? MAC showing on switch? | arp -a, switch port status |
| 3 – Network | Can I ping the IP? Is the route working? | ping, tracert |
| 4 – Transport | Is port 80/443 open? | telnet, nc |
| 7 – Application | Is web server or DNS working? | Browser, nslookup |
👉 You’re using OSI logic,
but testing TCP/IP protocols (Ping = ICMP, Telnet = TCP, DNS = UDP, etc.)
💬 5. Analogy
Imagine:
- TCP/IP = the car engine actually driving.
- OSI = the mechanic’s manual that helps you locate the problem (fuel system = Layer 1–2, engine = Layer 3–4, dashboard = Layer 7).
Without OSI, troubleshooting would be chaos — everyone would describe problems differently.
✅ 6. Summary
| Concept | OSI Model | TCP/IP Model |
|---|---|---|
| Main role | Diagnostic & teaching tool | Actual network operation |
| Used by | Humans (engineers) | Computers (software/hardware) |
| In troubleshooting | Layer-by-layer testing | Uses real TCP/IP commands (ping, nslookup, etc.) |
| In exams (e.g., CCNA) | To structure thinking | To understand real protocols |
💡 Final Takeaway
TCP/IP makes it work — OSI helps you fix it.
OSI = the map
TCP/IP = the journey
⚙️ The 4 vs 5 layer confusion explained
📘 Classic TCP/IP (4-layer version)
| TCP/IP Layer | What it includes | OSI Equivalent |
|---|---|---|
| Application | All software that creates/reads data (HTTP, DNS, SMTP, etc.) | Layers 5–7 (Application, Presentation, Session) |
| Transport | TCP, UDP, ports, reliability | Layer 4 |
| Internet | IP addressing, routing | Layer 3 |
| Network Access | Frames + physical media | Layers 1–2 |
📗 Modern textbooks (5-layer version)
Sometimes, they split Network Access into Data Link and Physical to align it more closely with OSI:
| TCP/IP Layer | Example protocols | OSI Equivalent |
|---|---|---|
| Application | HTTP, DNS | 7–5 |
| Transport | TCP, UDP | 4 |
| Network | IP, ICMP | 3 |
| Data Link | Ethernet, Wi-Fi, ARP | 2 |
| Physical | Copper, fiber, radio signals | 1 |
👉 Both are correct — it just depends on whether the author chooses to merge or separate the bottom layers.
🧠“How TCP/IP and OSI Work in Real Life”
Both the TCP/IP model and the OSI model are conceptual frameworks.
They define how devices should communicate across a network — no matter which brand, operating system, or hardware they use.
By following these standard communication rules (protocols), any two devices can exchange data reliably.
⚙️ History and Purpose
- TCP/IP was developed in the 1970s by the U.S. Department of Defense to make computers communicate over different networks (ARPANET → Internet).
- OSI was introduced in the 1980s by ISO to explain networking in more detail and provide a reference model for teaching, design, and troubleshooting.
- So, TCP/IP actually runs the Internet, while OSI helps us understand and diagnose it.
💻 How Devices Implement It
Inside every device, the CPU executes networking code written in the operating system’s network stack.
These software modules follow the TCP/IP layers, turning application data into packets that travel across the network.
When you search something in a browser:
| Layer (Conceptual) | What Happens in the Device | Examples / Details |
|---|---|---|
| Application | The browser (e.g., Chrome) sends your search request. | Data written in HTML/HTTP. |
| Transport | The OS’s transport module adds TCP (or UDP) headers, creates logical ports, and ensures reliability. | TCP = connection-oriented (e.g., HTTP), UDP = faster (e.g., DNS). |
| Internet | The IP module adds IP addresses, determines the route, and prepares the packet for delivery to the next hop (router). | IP, ICMP, ARP. |
| Network Access | Split into two functions: | |
| • Data Link software adds MAC addresses and frame info. | ||
| • Physical driver sends the bits as electrical signals (Ethernet) or radio waves (Wi-Fi). | Ethernet frames, Wi-Fi signals. |
At the destination, the same process happens in reverse, unwrapping the layers until the web page appears.
🧩 Summary
- Both OSI and TCP/IP are models.
- TCP/IP defines real, working protocols and software stacks.
- OSI defines how we explain, design, and troubleshoot those processes.
- Every device’s CPU, operating system, and network hardware cooperate layer-by-layer to make communication universal.
✅ In short:
TCP/IP makes networks work.
OSI makes networks understandable.
🌍 1. Why TCP/IP and OSI Are the Foundation of IT
The TCP/IP and OSI models are the language and blueprint of all digital communication.
Everything in IT — networking, cybersecurity, cloud, servers, virtualization, programming, IoT, even gaming —
depends on data moving between devices, and these two models explain exactly how that happens.
Without understanding them, you’re basically using technology without knowing how it actually works underneath.
⚙️ 2. How these models connect the entire IT world
| IT Field | How It Relates to OSI/TCP-IP |
|---|---|
| Networking | Configuring routers, switches, VLANs, IP addresses, firewalls — all based on TCP/IP layers. |
| Cybersecurity | Every security control (firewalls, SSL/TLS, VPNs, IPS) protects data at specific layers. |
| Cloud Computing | Cloud services (AWS, Azure, Google Cloud) depend on virtualized networks that follow TCP/IP. |
| Servers & DevOps | Web servers, load balancers, and containers all communicate via HTTP/TCP/IP. |
| Programming & APIs | Developers build apps that talk over TCP sockets, HTTP, or DNS — all defined by these models. |
| Virtualization | Virtual switches, virtual NICs, and SDN still obey OSI/TCP-IP logic. |
| Troubleshooting & Support | Diagnosing issues (ping, traceroute, DNS failure) means checking the right OSI/TCP-IP layer. |
Once you understand what happens at each layer,
you can pinpoint where any problem or function sits — hardware, software, or protocol.
🧱 3. Why beginners get lost without it
Without knowing the models:
- You wouldn’t know where data travels or why something fails.
- You’d mix up hardware vs. software issues.
- You couldn’t explain why pings work but websites don’t (Layer 3 vs Layer 7 issue).
- You’d rely only on trial-and-error instead of structured troubleshooting.
With it:
- You understand every part of the network stack.
- You can visualize how data flows from an app → cable → router → Internet → server.
- You can design, secure, and repair any network or system logically.
That’s why every certification — CCNA, CompTIA Network+, Security+, Azure, AWS —
starts with OSI and TCP/IP fundamentals.
💡 4. The perfect way to remember it
🧠 OSI teaches you how to think.
⚙️ TCP/IP shows you how it works.
Together, they form the blueprint of all modern IT systems.
✅ 5. Final takeaway for your students / followers
🚀 Without TCP/IP and OSI, there is no IT.
They are the foundation that explains every network, every device, every piece of communication in the digital world.
Master these, and you’ll understand everything else — from Wi-Fi to the cloud — with confidence.
@learntechfromzero TCP/IP and the OSI model explained in an easy way #tcp #osi #cybersecurity #networking #ccna ♬ MONTAGEM AZURE – Super Slowed – Nxxkz & Zyaki
