Two reference models describe the layered structure of networks. OSI has seven academic layers; TCP/IP has four practical ones. This chapter explains both, maps them to each other, and shows where every networking topic lives in the stack.
Memorize the order. The two classic mnemonics are 'All People Seem To Need Data Processing' (top down) and 'Please Do Not Throw Sausage Pizza Away' (bottom up).
| # | Layer | Job | Examples |
|---|---|---|---|
| 7 | Application | User-facing protocols | HTTP, DNS, SSH, SMTP |
| 6 | Presentation | Encoding, encryption, compression | TLS, JPEG, ASCII |
| 5 | Session | Open/close conversations | NetBIOS, RPC |
| 4 | Transport | End-to-end delivery, ports | TCP, UDP |
| 3 | Network | Logical addressing, routing | IPv4, IPv6, ICMP |
| 2 | Data Link | Frame on local segment, MAC | Ethernet, 802.11, ARP |
| 1 | Physical | Bits on the wire | Cabling, fiber, radio |
Real-world networks ran on TCP/IP long before OSI was finalized, and TCP/IP combines several OSI layers because no protocol on the Internet actually uses them separately. The four-layer TCP/IP model is what working engineers reach for in conversation.
| TCP/IP layer | Equivalent OSI layers | Examples |
|---|---|---|
| Application | 5–7 | HTTP, DNS, FTP, SSH |
| Transport | 4 | TCP, UDP |
| Internet | 3 | IPv4, IPv6, ICMP |
| Network access | 1–2 | Ethernet, 802.11, PPP |
1 more section in this chapter, plus a 3-question quiz.
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