OSI Model (7 Layers)

1. Physical — transmits raw bits over a medium.
2. Data Link — frames data, handles MAC addresses.
3. Network — routing via IP addresses.
4. Transport — reliable delivery; TCP vs UDP.
5. Session — manages sessions between applications.
6. Presentation — data formatting, encryption.
7. Application — HTTP, FTP, DNS, SMTP.

TCP vs UDP

TCP: Connection-oriented, reliable, ordered, slower. Used for web, email.
UDP: Connectionless, fast, no guarantee. Used for streaming, DNS, VoIP.

IP Addressing

IPv4: 32-bit, written as four octets (e.g. 192.168.1.1). Classes: A (0–127), B (128–191), C (192–223).