OSI vs TCP/IP Model
The OSI model is a 7-layer theoretical framework used to understand network communication. The TCP/IP model is the 4-layer protocol stack that actually runs the internet. Here's how they line up.
* ARP is sometimes placed at Layer 2.5 / Internet boundary depending on the source.
Key differences at a glance
- Layer count: OSI has 7 layers; TCP/IP has 4 (sometimes 5 if Physical and Data Link are split).
- Origin: OSI is an ISO/ITU-T standard from 1984; TCP/IP grew out of ARPANET in the 1970s and was standardised by the IETF.
- Purpose: OSI is descriptive — great for teaching and troubleshooting. TCP/IP is prescriptive — protocols that actually run on the wire.
- Strictness: OSI keeps layers tightly separated; TCP/IP layers are more pragmatic and overlap (e.g. HTTPS spans Application + Presentation + Session ideas).
- Real-world use: Every packet you send is TCP/IP; you reach for OSI when describing or isolating where a problem lives.
Encapsulation units (PDUs) per layer
| OSI Layer | PDU Name | Example |
|---|---|---|
| 7 Application | Data | HTTP GET / request body |
| 6 Presentation | Data | TLS-encrypted payload |
| 5 Session | Data | SMB session message |
| 4 Transport | Segment (TCP) / Datagram (UDP) | TCP segment with seq/ack |
| 3 Network | Packet | IPv4 packet with src/dst IP |
| 2 Data Link | Frame | Ethernet frame with MAC + FCS |
| 1 Physical | Bit | Voltage / light / radio pulse on the wire |
When to use which model
Use the OSI model when you want to explain a concept (e.g. "ARP works at Layer 2") or pinpoint where a fault occurs ("the cable is fine — must be a Layer 3 routing issue").
Use the TCP/IP model when you want to describe what's actually running — every modern protocol is part of the TCP/IP family.
In practice, network engineers slip between both models in the same sentence. CompTIA Network+ and CCNA exams expect fluency with both.
Want to go deeper?
Each OSI layer has its own page with interactive demos and Wireshark captures: