The OSI Model
The OSI (Open Systems Interconnection) Model is a conceptual framework that explains how data travels from one computer to another over a network.
It is divided into 7 layers, from the physical connection up to the application the user interacts with.
Think of the OSI Model as a troubleshooting ladder — you start from the bottom and move up until you find where things break.
Top = close to the user.
Bottom = close to the wire.
The 7 OSI layers (Top to Bottom)
Layer 7 - Application
Provides network services directly to applications used by humans.
This is what humans interact with.
Examples:
- HTTP / HTTPS (web)
- DNS
- FTP
- SMTP (email)
Key question:
“Is the application or service responding?”
Common Issues:
- Website not loading
- DNS not resolving
- Application service down
Layer 6 - Presentation
Handles data formatting, encryption, and compression.
“How is the data formatted?”
Examples:
- Encryption (TLS/SSL)
- Compression
- Data Encoding
Key question:
“Can both sides understand the data format?”
In practice, Layers 5–7 are often grouped together as the Application Layer.
Layer 5 - Session
Manages session setup, maintenance, and teardown.
Key question:
"Did the conversation stay open?"
or
"Are we still talking?"
This layer is mostly invisible in modern networking but still conceptually important.
Layer 4 - Transport
Controls how data is delivered, including reliability and ports.
Protocols:
TCP – reliable, ordered (web, email)
UDP – fast, no guarantees (DNS, streaming)
Key question:
“Which port is used, and is the connection reliable?”
Common issues:
- Port blocked by firewall
- Service listening on the wrong port
- TCP handshake failure
Today, I learned about the OSI Model and how it explains the process of data communication in a network. I learned that it has 7 layers, and each layer has a specific role, from user interaction down to data delivery. I also learned that the OSI Model can be used as a troubleshooting guide to identify where network problems occur. This lesson helped me better understand how networking works in an organized and systematic way.
The layer 3 to 1 (Network, Data Link and Physical) will be continued tomorrow.
The Continuation;
Layer 3 - Network
Handles logical addressing and routing between networks.
Concepts:
- IP addresses
- Subnetting
- Routing
- ICMP (ping)
Key question:
"Do I know where to send this packet?"
Common problems:
- Wrong IP
- Bad subnet
- No route to host
Layer 2 - Data Link
Handles local network delivery using hardware addresses.
Concepts:
- MAC addresses
- ARP
- Switches
- VLANs
Key question:
“Who exactly owns this IP on my local network?”
Common problems:
- ARP issues
- Switch misconfig
- Wrong VLAN
Layer 1 – Physical
Moves raw bits across physical media.
Examples:
- Ethernet cables
- Wi-Fi signals
- Fiber optics
Key question:
“Is there a physical connection?”
Common problems:
- Bad cable
- Wi-Fi signal weak
- Interface down
Step by step example using application like Facebook messenger
How Facebook Messenger works using the OSI Model?
Application Layer
You type your message in the messenger chat box with your friend like "Hi, how are you?", and the app understand that you want to sent a message to your friend. This is the layer you directly interact with as a user.
Presentation Layer
The message is translated into suitable format for sending and protected so it stays secured while being sent.
Session Layer
Messenger makes sure the conversation with your friend is active and ready to continue.
Transport Layer
When you hit send, this layer decides how to transport your data:
The message is divided into smaller parts and delivered reliably to ensure complete transmission.
Network layer
The destination is identified using IP Address and the appropriate path is selected for the message.
Data Link Layer
The message is transferred to the next network device within the local network (like Wi-Fi).
Physical Layer
The message is sent as signals through the air or cables until it reaches your friend’s phone.
Today, I learned more about the OSI Model, focusing on the lower layers: Network, Data Link, and Physical. The Network layer handles logical addressing and routing using IP addresses and determines the path for data. The Data Link layer delivers data locally using MAC addresses and devices like switches. The Physical layer transmits raw bits over cables, fiber, or Wi-Fi. Using Facebook Messenger as an example, I saw how a message is prepared, divided, routed, and sent as signals until it reaches the recipient, which helped me understand how these layers work together in real networks.
Top comments (0)