OSI vs TCP/IP
A seven-layer teaching model versus a four-layer running model. Same ideas, different cuts.
Level: beginner
What is it?
OSI (ISO/IEC 7498) specified a full protocol suite that lost. Its seven-layer map survived as a language. TCP/IP won as the implementation. The functions (addressing, hops, processes, syntax, applications) exist in both; they are grouped differently.
Why does it matter?
Exam questions love the mapping. Real work loves knowing that TLS is not a kernel 'presentation module' and that a multilayer switch blurs L2/L3 on purpose.
Mental model
Two ways to slice the same stack of wrappers around an HTTP body.
How it works
- OSI: 7 layers, strict service definitions, a failed protocol suite.
- TCP/IP: 4 layers, messy, deployed.
- L5/L6 in OSI are real jobs (sessions, encodings, crypto) done inside apps on the internet.
- Troubleshooting still uses OSI names: 'is this L1 or L3?'
Real-world usage
- Interviews
- Network certifications
- Incident communication
Common mistakes
- Saying OSI is deprecated so you can skip layer names
- Mapping QUIC to a single OSI layer — it straddles
Interview questions
Give the mapping.
App = 7–5, transport = 4, internet = 3, link = 2–1.
Name a protocol that punches holes in strict layering.
NAT (L3/L4 rewrite), QUIC (transport+crypto+streams), TLS-terminating proxies.
| Question | OSI answer | TCP/IP answer |
|---|---|---|
| Where is HTTP? | L7 | Application |
| Where is TLS? | Often taught as L6 | Application over TCP |
| Where is TCP? | L4 | Transport |
| Where is IP? | L3 | Internet |
| Where is Ethernet? | L2 | Link |
| Where is the cable? | L1 | Link / PHY |