OSI Model Deep Dive with Real Data Flow and Device Roles

In this blog you will learn the core concept of how data is transferred from one computer to another in a network through OSI model. It includes step-by-step data transformation across the 7 layers of OSI, involvement of switches and routers, and how data looks at each step.

Overview of the OSI Model

| Layer | Name         | Function                              | Example Protocols |
| ----- | ------------ | ------------------------------------- | ----------------- |
| 7     | Application  | End-user app interaction              | HTTP, SMTP        |
| 6     | Presentation | Encryption, formatting, compression   | TLS, SSL          |
| 5     | Session      | Manages sessions and connections      | NetBIOS, RPC      |
| 4     | Transport    | Reliable delivery, segmentation       | TCP, UDP          |
| 3     | Network      | Logical addressing and routing        | IP, ICMP          |
| 2     | Data Link    | Physical addressing (MAC) and framing | Ethernet          |
| 1     | Physical     | Transmission of bits over the medium  | Cables, Wi-Fi     |

Full End-to-End Data Transfer (Computer 1 → Computer 2)

Scenario:

• Computer 1 IP: 192.168.1.2, MAC: AA:AA:AA:AA:AA:AA
• Computer 2 IP: 192.168.2.2, MAC: BB:BB:BB:BB:BB:BB
• Router MAC (in): 11:11:11:11:11:11, MAC (out): 22:22:22:22:22:22

Devices:

• Switch 1 (Layer 2 device) between Computer 1 and Router
• Router (Layer 3 device) between Switch 1 and Switch 2
• Switch 2 (Layer 2 device) between Router and Computer 2

Computer 1: Data Creation (All 7 Layers)

1. Application Layer (L7)

• Data: ”Hello, Computer 2!”

2. Presentation Layer (L6)

• Encrypts or formats data (e.g., SSL/TLS)
• Output: Encrypted Data Blob

3. Session Layer (L5)

• Opens a session if needed (e.g., via cookies, handshakes)

4. Transport Layer (L4)

• Adds TCP header (port numbers, sequence, ACK)
• Output: TCP Segment

5. Network Layer (L3)

• Adds IP header with:
• Source IP: 192.168.1.2
• Destination IP: 192.168.2.2
• Output: IP Packet

6. Data Link Layer (L2)

• Adds MAC header with:
• Source MAC: AA:AA:AA:AA:AA:AA
• Destination MAC: 11:11:11:11:11:11 (Router MAC)
• Output: Ethernet Frame

7. Physical Layer (L1)

• Transmits bits over the cable to Switch 1

Switch 1 (Layer 2 Device)

• Reads only the MAC header (Layer 2)
• Forwards the frame based on MAC table
• Does not modify the frame
• Sends the frame to the Router

Router (Layer 3 Device)

1. Receives the Ethernet Frame

• MAC Destination = Router’s MAC → Accepts it

2. Strips Layer 2 Header (Ethernet)

• Keeps the IP Packet (L3 and above remain unchanged)

3. Reads Destination IP (192.168.2.2)

• Finds next hop (Computer 2 in this case)

4. Uses ARP to find MAC of Computer 2 → BB:BB:BB:BB:BB:BB

5. Builds a New Ethernet Frame:

• Source MAC: 22:22:22:22:22:22 (Router’s outgoing interface)
• Destination MAC: BB:BB:BB:BB:BB:BB
• Wraps original IP Packet

6. Sends new frame to Switch 2

Switch 2 (Layer 2 Device)

• Reads Layer 2 (Ethernet frame)
• Forwards based on MAC table to Computer 2
• Does not alter the frame

Computer 2: Data Reception (Layer 1 → Layer 7)

1. Physical Layer (L1)

• Receives the raw bits

2. Data Link Layer (L2)

• Reads MAC header
• Destination MAC matches → Accepts the frame
• Strips MAC header

3. Network Layer (L3)

• Reads IP header
• Destination IP matches → Accepts the packet
• Strips IP header

4. Transport Layer (L4)

• Uses TCP header to reassemble and manage data

5. Session Layer (L5)

• Maintains/ends the session

6. Presentation Layer (L6)

• Decrypts and decodes the data

7. Application Layer (L7)

• Presents ”Hello, Computer 2!” to the user/app

Summary Table of Data Formats at Each Layer

| Layer | Sender                         | Router (Intermediate)              | Receiver                     |
| ----- | ------------------------------ | ---------------------------------- | ---------------------------- |
| L1    | Bits                           | Bits                               | Bits                         |
| L2    | Ethernet Frame (MAC to Router) | Ethernet Frame (MAC to Computer 2) | Ethernet Frame (MAC to Self) |
| L3    | IP Packet                      | IP Packet (same)                   | IP Packet                    |
| L4    | TCP Segment                    | TCP Segment                        | TCP Segment                  |
| L5    | Session Data                   | Session Data                       | Session Data                 |
| L6    | Encrypted Data                 | Encrypted Data                     | Decrypted Data               |
| L7    | Message: "Hello"               | -                                  | Message: "Hello"             |

Key Takeaways

• Computers implement all 7 OSI layers to fully understand and generate end-to-end communication.
• Switches work at Layer 2, forward frames based on MAC addresses.
• Routers work at Layer 3, route packets based on IP addresses.
• Only routers and end devices read IP addresses — switches do not.
• MAC address changes hop-by-hop, IP address stays constant.
• Each layer encapsulates the data, and the reverse happens on the receiving end.

Hope you understood how actually data transfer from one computer to another work under the hood in a network.