What is the OSI model?

The OSI (Open Systems Interconnection) model is a conceptual framework that standardizes the functions of a communication system into 7 layers, allowing different systems and vendors to communicate. From top to bottom: Layer 7 — Application (HTTP, FTP, SMTP, DNS — user-facing services), Layer 6 — Presentation (data format translation, encryption, compression), Layer 5 — Session (managing sessions/connections between applications), Layer 4 — Transport (TCP/UDP, end-to-end communication, port numbers), Layer 3 — Network (IP addressing, routing), Layer 2 — Data Link (MAC addresses, framing, switches), Layer 1 — Physical (cables, signals, bits). Mnemonic: "All People Seem To Need Data Processing" (top to bottom).