TCP/IP Model
Four layers the internet actually runs: application, transport, internet, network access.
Level: beginner
What is it?
The DoD / Internet model has four layers: Application (OSI 7–5), Transport (OSI 4), Internet (OSI 3), Network Access / Link (OSI 2–1). IP sits in the internet layer; TCP/UDP in transport; HTTP/DNS in application. This is what RFCs and kernels implement.
Why does it matter?
OSI is the vocabulary; TCP/IP is the running system. You need both: one to talk, one to read code and packet captures.
Mental model
The same sandwich, cut into four layers instead of seven. Presentation and session are eaten by the application.
How it works
- Application: HTTP, DNS, SSH, TLS…
- Transport: TCP, UDP, QUIC.
- Internet: IP, ICMP, routing.
- Link: Ethernet, Wi-Fi, ARP, PPP, the PHY.
- Encapsulation: data → segment → packet → frame → bits. Same story as OSI.
Real-world usage
- RFCs
- Linux networking stack
- Wireshark’s layering
Common mistakes
- Treating the two models as competing religions
- Forgetting that 'link' includes both MAC and PHY
Interview questions
Map OSI to TCP/IP.
7–5 → application, 4 → transport, 3 → internet, 2–1 → link/network access.
Which model does the internet run?
TCP/IP. OSI is the teaching and troubleshooting vocabulary.
| TCP/IP | OSI | Examples |
|---|---|---|
| Application | 7 + 6 + 5 | HTTP, TLS, DNS, SMTP |
| Transport | 4 | TCP, UDP, QUIC |
| Internet | 3 | IP, ICMP, OSPF, BGP |
| Network access | 2 + 1 | Ethernet, Wi-Fi, optics |