What is the OSI Model?
OSI (Open Systems Interconnection) is a conceptual framework that divides network communication into seven distinct layers. Each layer handles one job and talks only to the layers directly above and below it.
01Why layers?
Splitting the problem means you can change one piece without touching the rest. When HTTPS replaced HTTP, only the Application layer changed — the routing, switching, and cabling below it didn't move. That's the power of layering.
- Isolation — a bug in TCP doesn't crash the Wi-Fi driver
- Interoperability — a Cisco router and a Juniper router speak the same Layer 3
- Troubleshooting — "ping works, HTTP doesn't" instantly narrows the problem to Layer 7
02The seven layers
Read top-down: data starts at Application and gets wrapped at every step on the way down to the wire. At the destination the process reverses — each layer strips its own header and passes the remainder up.
7
Application
User-facing services — the only layer your browser touches directly
HTTPDNSSMTPFTPSSH
6
Presentation
Data format, encryption, compression — translates between app and network
TLSJPEGASCII
5
Session
Opens, manages, and closes sessions between two applications
NetBIOSRPC
4
Transport
End-to-end delivery, segmentation, error recovery, flow control
TCPUDP
3
Network
Logical addressing and routing — how packets cross multiple networks
IPICMPBGP
2
Data Link
Node-to-node delivery on the same network, MAC addressing, error detection
EthernetWi-FiARP
1
Physical
Raw bits on the medium — voltage, light pulses, or radio waves
Ethernet cableFiberWi-Fi signal
03Memory trick
Top to bottom · layers 7 → 1
All
People
Seem
To
Need
Data
Processing
Application · Presentation · Session · Transport · Network · Data Link · Physical
04Summary
| Layer | Name | Key idea |
|---|---|---|
| 7 | Application | What the user sees |
| 6 | Presentation | Format & encryption |
| 5 | Session | Connection lifecycle |
| 4 | Transport | Ports, TCP/UDP |
| 3 | Network | IP addresses, routing |
| 2 | Data Link | MAC, same-network hop |
| 1 | Physical | Bits on the wire |