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System Design Masterclass · Cloud Infrastructure
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Here's a dirty secret about networking: the OSI model was never fully implemented. Nobody runs a pure OSI stack in production. The internet runs on /IP, which predates OSI and doesn't map to it cleanly.
So why does every networking course, certification exam, and system design interview still ask about it?
Because OSI is a thinking tool. Its seven layers give you a shared vocabulary for talking about where in the networking stack a problem lives. When a network engineer says "this is a Layer 7 issue," everyone in the room instantly knows it's an application-level problem. When someone says "check Layer 3," you know they're talking about routing.
The OSI model didn't win the implementation war. TCP/IP did. But it absolutely won the conversation war. You'll use it to communicate, debug, and reason about systems even though the actual bits on the wire follow a different model.
| Layer | Name | What It Does | Example Protocols/Tech | /IP Equivalent |
|---|---|---|---|---|
| 7 | Application | User-facing protocols | HTTP, SMTP, , FTP | Application |
| 6 | Presentation | Data formatting, encryption, compression | SSL/TLS, JPEG, JSON | Application |
| 5 | Session | Connection management, checkpointing | NetBIOS, RPC | Application |
| 4 | Transport | End-to-end delivery, flow control | TCP, UDP | Transport |
| 3 | Network | Logical addressing and routing | IP, ICMP, OSPF | Internet |
| 2 | Data Link | Physical addressing, error detection | Ethernet, Wi-Fi, ARP | Network Access |
| 1 | Physical | Raw bits over physical media | Cables, fiber, radio waves | Network Access |
The classic mnemonic is "Please Do Not Throw Sausage Pizza Away" (from bottom to top). Or, from top to bottom: "All People Seem To Need Data Processing."
Where OSI and TCP/IP diverge: OSI splits the TCP/IP Application layer into three (Application, Presentation, Session) and splits Network Access into two (Data Link, Physical). In practice, modern applications blur these boundaries. TLS handles both Presentation and Session concerns, and /2 manages sessions at the Application level.