Topic 3 · Networks

Two or more devices that can exchange data. Whether wired or wireless, scale doesn't matter.

Personal Area Network. ~10 m. Phone + earbuds.

Local Area Network. ~100 m. Home/office/school. One organisation.

Metropolitan. Kilometres. Across a city.

Wide Area. Countries/global. The Internet is the largest WAN.

All devices → central hub/switch. Most common. Hub fail = network dead.

One shared cable. Cheap. One break kills all.

Loop. Predictable. One device down breaks ring.

Many connections. Redundant + expensive. Used in backbone routers.

Dedicated server provides services; clients request. Centralised. Single point of failure unless replicated.

Every peer = client + server. Decentralised. Resilient. BitTorrent, Bitcoin.

Hub = dumb broadcast. Switch = smart, within LAN (L2, MAC). Router = between networks (L3, IP).

Internet = the network. Web (HTTP/HTML) = one service that runs on it. Email, gaming also on the Internet.

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).

~10 m, low power, 1–50 Mbps. PAN. Earbuds, peripherals. 2.4 GHz.

~50–100 m, medium power, 100 Mbps–9 Gbps. LAN. Home/office. 2.4/5/6 GHz.

km via towers, high power, 100 Mbps–Gbps. WAN. Mobile. Hand-off between cells.

Higher freq = more bandwidth BUT shorter range + worse wall penetration.

Confidentiality (no peeking) · Integrity (no tampering) · Availability (reachable).

Same key both sides. Fast. AES, ChaCha20. Key-distribution problem.

Public key encrypts; private key decrypts. RSA, ECC. ~1000× slower. Solves key distribution.

Asymmetric for the handshake (share session key) → symmetric AES for bulk data. Best of both.

Encrypt hash with PRIVATE key; verify with PUBLIC key. Proves origin + integrity.

Site's public key signed by a Certificate Authority (CA). Browser verifies → green padlock.

Multi-factor: KNOW (password) + HAVE (phone) + ARE (fingerprint). Defeats stolen passwords.

Hashing = one-way (for passwords, with salt + bcrypt/Argon2). Encryption = reversible.

MITM · phishing · DoS/DDoS · malware · SQL injection · XSS · brute force.

HTTPS encrypts CONNECTION. Phishing sites can have valid HTTPS too. Always check URL.