Networking tricks that make the OSI model stick

OSI (Open Systems Interconnection) model layers 7 to 1: All People Seem To Need Data Processing (A=Application, P=Presentation, S=Session, T=Transport, N=Network, D=Data Link, P=Physical)

7Application — HTTP, FTP, DNS

6Presentation — encryption, compression

5Session — connections

4Transport — TCP/UDP, ports

3Network — IP addressing

2Data Link — MAC, switches

1Physical — cables, signals

TCP (Transmission Control Protocol): reliable, ordered, error-checked. UDP (User Datagram Protocol): fast, no guarantees.

TCP vs UDP — TCP: connection-oriented, guaranteed delivery and order — web, email, file transfer. UDP: connectionless, no guarantee, lower latency — video streaming, gaming, DNS.

DNS (Domain Name System): domain name to IP address — the internet's phone book

DNS — You type google.com → DNS resolver queries root → TLD → authoritative server → returns IP. Cached at multiple levels. Without DNS you'd memorize IP addresses.

IP classes: A (1-126 huge networks), B (128-191), C (192-223 small networks)

IP Address Classes — Class A: 1–126, up to 16M hosts (large orgs). Class B: 128–191, up to 65K hosts. Class C: 192–223, up to 254 hosts (small networks). 127.0.0.1 = localhost.

HTTP methods: GET (read), POST (create), PUT (update/replace), DELETE (delete) — REST basics

HTTP Methods — GET: retrieve resource (safe, idempotent). POST: create new resource (not idempotent). PUT: update/replace resource (idempotent). DELETE: remove resource. PATCH: partial update. Memorize these for any web development or API work.

IP addressing: IPv4 = 32 bits (4.3 billion addresses). IPv6 = 128 bits (340 undecillion addresses).

IPv4 vs IPv6 — IPv4: 32-bit addresses like 192.168.1.1 — only ~4.3 billion unique addresses. Running out since 1990s. NAT (Network Address Translation): multiple devices share one public IP. IPv6: 128-bit addresses like 2001:0db8:85a3::8a2e:0370:7334 — essentially unlimited. Adoption slow due to compatibility issues.

Subnetting: divide network into smaller networks. Subnet mask determines network vs host portion.

Subnetting — Subnet mask: 255.255.255.0 = /24 means first 24 bits are network, last 8 bits are hosts (254 usable hosts). CIDR notation: 192.168.1.0/24. Subnetting: divide large network into smaller ones for security and efficiency. Calculate: network address, broadcast address, host range.

Firewall: filters network traffic based on rules. Stateful: tracks connection state. Stateless: each packet independent.

Firewalls — Packet filtering: check source/destination IP and port against rules. Stateful inspection: tracks active connections — knows if packet is part of established session. Application layer: inspect content (Layer 7). DMZ (demilitarized zone): semi-trusted network between internet and internal network.

HTTP (HyperText Transfer Protocol) vs HTTPS: HTTPS = HTTP + TLS (Transport Layer Security) encryption. Port 80 vs 443. Certificate verifies server identity.

HTTP vs HTTPS — HTTP: plaintext — anyone between you and server can read traffic. HTTPS: HTTP + TLS (Transport Layer Security). TLS handshake: client and server negotiate encryption, server presents certificate (signed by trusted CA), establish symmetric session key. Certificate: verifies server is who it claims to be. Always use HTTPS.

Latency vs bandwidth: latency = delay (milliseconds). Bandwidth = capacity (Mbps). Latency limits interactive apps.

Latency vs Bandwidth — Latency (RTT - round trip time): time for a packet to travel to destination and back. Affected by: physical distance, number of hops, congestion. Low latency critical for: gaming, video calls, trading. Bandwidth: maximum data transfer rate. Throughput: actual achieved rate. High bandwidth doesn't help if latency is high.

CDN (Content Delivery Network) — serve content from servers geographically close to users

Content Delivery Networks — CDN: distributed network of servers that cache static content (images, CSS, JS, video) geographically near users. Request goes to nearest CDN edge server instead of origin server. Reduces latency, reduces load on origin, improves availability. Cloudflare, Akamai, AWS CloudFront. Used by Netflix, YouTube, Amazon.

SSH (Secure Shell) — encrypted remote access. Port 22. Public/private key authentication safer than passwords.

SSH and Remote Access — SSH: encrypted protocol for remote command-line access. Replaced Telnet (plaintext). Key-based authentication: generate public/private key pair. Public key placed on server. Private key stays on client. More secure than passwords — can't be brute-forced if key is long enough. Port forwarding: tunnel other protocols through SSH.