OSI vs TCP/IP — The 60-Second Map

Networking is often explained with the OSI model, a neat seven-layer diagram you've probably seen in textbooks.

Real networks, however, usually follow the TCP/IP model, which condenses those layers into four practical ones.

The Comparison

OSI Layer	Typical Purpose	TCP/IP Equivalent	Example Protocols
7 Application	User programs	Application	HTTP, DNS, SSH
6 Presentation	Data formatting	(often folded in)	TLS, MIME
5 Session	Connection state	(folded into Transport)	RPC
4 Transport	End-to-end delivery	Transport	TCP, UDP
3 Network	Routing packets	Internet	IPv4, IPv6, ICMP
2 Data Link	Local delivery	Link	Ethernet, ARP
1 Physical	Signals and media	(host hardware)	Cables, Wi-Fi

In Practice

• Layers 1–2 (move bits inside a local network) → handled by hardware and drivers
• Layers 3–4 (route and deliver packets end-to-end) → that's our focus
• Layers 5–7 (applications) → we'll build simple examples (HTTP server, DNS responder)

What You Actually See

When you inspect a packet in Wireshark or print your own debug trace, you'll recognize:

Ethernet (L2)
 └─ IP (L3)
 └─ TCP or UDP (L4)
 └─ Application data (L7)

Info: Those are the headers you'll parse, modify, and generate by hand. By the end of the project you'll be able to look at any packet capture and identify exactly which layer produced each byte.