"Please Do Not Throw Sausage Pizza Away"
You need to memorize the order. The standard mnemonic reads from bottom (Layer 1) to top (Layer 7): Please Do Not Throw Sausage Pizza Away. Each word maps to a layer.
| Layer | Name | Unit | Key Protocols / Hardware |
|---|---|---|---|
| 7 | Application | Data / Message | HTTP, HTTPS, SSH, DNS, SMTP |
| 6 | Presentation | Data | TLS/SSL, JPEG, ASCII, gzip |
| 5 | Session | Data | Sockets, NetBIOS, RPC |
| 4 | Transport | Segment | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, Routers |
| 2 | Data Link | Frame | Ethernet, ARP, Switches, MAC |
| 1 | Physical | Bit | Cables, Fiber, WiFi, Hubs |
Two important conventions to know:
Layers 1–4 are "lower layers" — they deal with how data moves. Engineers working on infrastructure, networking, and security spend most of their time here.
Layers 5–7 are "upper layers" — they deal with what data means. Application developers and API engineers spend most of their time here.
In practice, the TCP/IP model (the actual protocol suite the internet runs on) collapses the OSI model into four layers: Network Access (OSI 1–2), Internet (OSI 3), Transport (OSI 4), and Application (OSI 5–7). The OSI model is the conceptual framework; TCP/IP is the implementation. You need to understand both and know how they relate.