Topic 3 · Networks

Agreed RULES for communication. Specifies format + order + error handling.

Data fragment + header (src+dst addr, sequence) + footer (error-check).

Messages split into packets, each routed independently, reassembled by sequence.

Dedicated line per call (old telephone). Wasteful + fragile compared to packet switching.

Reliable, ordered, slower. 3-way handshake (SYN/SYN-ACK/ACK). For web, email, files.

Fast, no guarantees. For video/voice/gaming/DNS — anywhere freshness beats reliability.

HTTP (80) · HTTPS (443) · DNS (53) · FTP (21) · SMTP (25) · SSH (22).

Physical · Data Link · Network · Transport · Session · Presentation · Application. (Please Do Not Throw Sausage Pizza Away.)

Link · Internet · Transport · Application. What the real Internet uses.

Each layer wraps in own header (and sometimes footer). Reversed on receive.

MAC = same LAN. IP = whole Internet. Port = which app on the device.

Router = L3 (Network/IP). Switch = L2 (Data Link/MAC).