Networking Fundamentals — How Data Travels

What Is a Network?

A system that connects computers so they can exchange data.

Classification by scale:

LAN (Local Area Network): within a building or campus
WAN (Wide Area Network): across cities, countries, or continents
Internet: the global network of networks

The OSI 7-Layer Model

A standard model that divides network communication into 7 abstract layers:

Application Layer: HTTP, FTP, DNS
Presentation Layer: Encryption, encoding
Session Layer: Connection management
Transport Layer: TCP, UDP — port numbers
Network Layer: IP — routing, IP addresses
Data Link Layer: MAC addresses, Ethernet
Physical Layer: Electrical signals, fiber optics

TCP vs. UDP

TCP (Transmission Control Protocol):

→ Connection-oriented: 3-way handshake (SYN → SYN-ACK → ACK)

→ Reliable: guarantees packet order, retransmits on loss

→ Flow control, congestion control

→ Slower but stable

Used for: web (HTTP), email, file transfer

UDP (User Datagram Protocol):

→ Connectionless: sends immediately

→ Unreliable: no guarantee of order or delivery

→ Fast, low overhead

Used for: streaming, gaming, DNS, VoIP

IP Addresses and Routing

IPv4: 32-bit (e.g., 192.168.1.1) — ~4.3 billion addresses

IPv6: 128-bit (e.g., 2001:db8::1) — effectively unlimited

Private IP addresses:

192.168.x.x / 10.x.x.x / 172.16–31.x.x
Cannot communicate directly on the internet → NAT required

Routing:

Routers inspect the destination IP in each packet and decide the next hop
Based on a routing table
BGP: the protocol routers use to exchange paths across the internet

DNS (Domain Name System)

Role: translates domain names into IP addresses
Example: google.com → 142.250.80.46

DNS lookup process

Check browser cache
Check OS cache → /etc/hosts file
Query local DNS server (provided by ISP)
Root DNS → TLD DNS (.com) → Authoritative DNS
IP address returned → cached for future use

Common DNS record types

A: domain → IPv4 address
AAAA: domain → IPv6 address
CNAME: domain alias
MX: mail server

HTTP / HTTPS

HTTP (HyperText Transfer Protocol):

The foundation of web communication
Stateless: each request is independent
Default port 80

HTTP methods:

GET: Retrieve a resource
POST: Create a resource
PUT/PATCH: Modify a resource
DELETE: Delete a resource

HTTP status codes:

200 OK: Success
301/302: Redirect
400 Bad Request: Client error
401 Unauthorized: Authentication required
403 Forbidden: Access denied
404 Not Found: Resource doesn’t exist
500 Internal Server Error: Server error

HTTPS:

HTTP + TLS/SSL encryption
Server identity verified via certificate
Default port 443

HTTP/1.1 vs. HTTP/2 vs. HTTP/3

HTTP/1.1: One TCP connection per request (head-of-line blocking)
HTTP/2: Multiplexing over a single connection (parallel requests)
HTTP/3: QUIC protocol over UDP (lower latency)

Key Takeaways

TCP: reliable, ordered delivery, slower / UDP: fast, unreliable OSI 7 layers (bottom to top): Physical, Data Link, Network, Transport, Session, Presentation, Application DNS: translates domain names to IP addresses (the internet’s phone book) HTTP status codes: 2xx success, 3xx redirect, 4xx client error, 5xx server error