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:

7. Application Layer:     HTTP, FTP, DNS
6. Presentation Layer:    Encryption, encoding
5. Session Layer:         Connection management
4. Transport Layer:       TCP, UDP — port numbers
3. Network Layer:         IP — routing, IP addresses
2. Data Link Layer:       MAC addresses, Ethernet
1. 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:
1. Check browser cache
2. Check OS cache → /etc/hosts file
3. Query local DNS server (provided by ISP)
4. Root DNS → TLD DNS (.com) → Authoritative DNS
5. 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)

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