<< back to Guides
🌐 Quick Guide to Network Layers (OSI Model)
The OSI (Open Systems Interconnection) model divides network communication into 7 layers to standardize interactions between systems.
📶 OSI Model Overview
| Layer | Name | Function | Protocol Examples |
|---|---|---|---|
| 7 | Application | End-user services, APIs | HTTP, FTP, SMTP, DNS |
| 6 | Presentation | Data formatting, encryption, compression | SSL/TLS, JPEG, ASCII |
| 5 | Session | Session management (start/stop/resume) | NetBIOS, RPC, PPTP |
| 4 | Transport | Reliable transmission, flow control | TCP, UDP |
| 3 | Network | Routing, addressing | IP, ICMP, ARP |
| 2 | Data Link | Node-to-node transfer, MAC addressing | Ethernet, PPP, Frame Relay |
| 1 | Physical | Bit transmission through medium | Cables, Fiber, Wi-Fi, Hubs |
🔍 Layer Breakdown
🧑💻 7. Application Layer
Interface between user and network services.
Browser ──▶ HTTP ──▶ Server
- Email (SMTP), web browsing (HTTP), DNS resolution
🧰 6. Presentation Layer
Data translation, encryption/decryption, compression.
- Converts from binary to human-readable and vice versa
- Applies TLS for secure sessions
🧭 5. Session Layer
Establishes, manages, and terminates sessions.
- Useful in stateful protocols (e.g., NetBIOS, SMB)
📦 4. Transport Layer
Reliable or fast delivery. Adds ports, retransmits lost packets.
- TCP: reliable, ordered (web, email)
- UDP: faster, no guarantee (video, games)
TCP: 3-way handshake → SYN, SYN-ACK, ACK
UDP: Send and hope it arrives
🌍 3. Network Layer
Determines best path for packet delivery.
- IP addressing and routing
- ICMP for error messaging (e.g., ping)
📡 2. Data Link Layer
Direct node-to-node delivery over a physical link.
- Adds MAC addresses
- Detects and sometimes corrects physical layer errors
MAC address: 00:1A:2B:3C:4D:5E
⚙️ 1. Physical Layer
Converts data into electrical/optical signals.
- Medium: Ethernet cable, Wi-Fi, fiber optics
- Defines voltages, pinouts, etc.
🧠 Mnemonic
"Please Do Not Throw Sausage Pizza Away"
(Physical, Data Link, Network, Transport, Session, Presentation, Application)
🔀 TCP/IP vs OSI
| OSI Layer | TCP/IP Equivalent |
|---|---|
| Application (7-5) | Application |
| Transport (4) | Transport |
| Network (3) | Internet |
| Data/Physical | Network Access / Link |
✅ Key Takeaways
- OSI helps structure communication protocols.
- Understanding the layers helps with debugging and optimization.
- Most practical networking focuses on Layers 1–4.