Networking & HTTP — How the Internet Works

Why Networking Fundamentals Matter

Every web application, API call, and cloud service depends on the network. Understanding how data travels from a browser to a server and back is essential for debugging issues, optimizing performance, and designing robust distributed systems.

Why this matters for your career:

Network issues are among the hardest to debug — understanding the stack helps
HTTP/HTTPS knowledge is required for web development, API design, and security
DNS misconfiguration is a common cause of downtime
TLS/SSL certificates are essential for production deployments
Interviewers often ask about TCP, DNS, and what happens when you type a URL in a browser

What Happens When You Type a URL in a Browser?

You type https://www.example.com in the browser
Browser checks its cache for the DNS record of www.example.com
If not cached, browser queries a DNS resolver (usually your ISP's DNS server)
DNS resolver recursively queries root → TLD → authoritative nameservers
Browser gets the IP address (e.g., 93.184.216.34)
Browser establishes a TCP connection to the IP on port 443 (HTTPS)
TLS handshake occurs to negotiate encryption
Browser sends an HTTP GET request
Server processes the request and returns an HTTP response
Browser renders the HTML, fetches additional resources (CSS, JS, images)

The OSI Model

The OSI model describes network communication in 7 layers:

| Layer | Number | Function | Example Protocols | |-------|--------|----------|-------------------| | Physical | 1 | Raw bit transmission over wire/air | Ethernet, Wi-Fi | | Data Link | 2 | Framing, MAC addresses | Ethernet, ARP | | Network | 3 | Routing, IP addressing | IP, ICMP | | Transport | 4 | End-to-end reliability | TCP, UDP | | Session | 5 | Session management | TLS (partially) | | Presentation | 6 | Data encoding, encryption | TLS, SSL | | Application | 7 | User-facing protocols | HTTP, FTP, DNS, SMTP |

In practice, TCP/IP networking combines layers 3-4 (TCP/IP) and layers 5-7 are often grouped together.

TCP vs. UDP

| Feature | TCP | UDP | |---------|-----|-----| | Connection | Connection-oriented (3-way handshake) | Connectionless | | Reliability | Guaranteed delivery, retransmission | Best-effort, no guarantee | | Ordering | In-order delivery | No ordering | | Flow control | Yes (sliding window) | No | | Congestion control | Yes | No | | Speed | Slower (overhead) | Faster (no overhead) | | Use cases | HTTP, HTTPS, SSH, SMTP, FTP | DNS, VoIP, video streaming, gaming |

TCP Three-Way Handshake

Client                     Server
  |    SYN (Seq=x)           |
  |------------------------->|
  |    SYN+ACK (Seq=y, Ack=x+1) |
  |<-------------------------|
  |    ACK (Seq=x+1, Ack=y+1)   |
  |------------------------->|
  |     Connection Established    |

This handshake adds one round-trip time (RTT) to every new HTTP connection, which is why HTTP/2 and HTTP/3 use connection multiplexing.

HTTP Protocol

HTTP Methods

| Method | Purpose | Idempotent | Safe | |--------|---------|------------|------| | GET | Retrieve a resource | Yes | Yes | | POST | Create a resource | No | No | | PUT | Update/replace a resource | Yes | No | | PATCH | Partial update | No | No | | DELETE | Remove a resource | Yes | No | | HEAD | Get headers only | Yes | Yes | | OPTIONS | Check available methods | Yes | Yes |

HTTP Status Codes

| Code Range | Category | Example | |------------|----------|--------| | 1xx | Informational | 101 Switching Protocols (WebSocket) | | 2xx | Success | 200 OK, 201 Created | | 3xx | Redirection | 301 Moved Permanently, 304 Not Modified | | 4xx | Client Error | 404 Not Found, 401 Unauthorized | | 5xx | Server Error | 500 Internal Server Error, 503 Service Unavailable |

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

| Feature | HTTP/1.1 | HTTP/2 | HTTP/3 | |---------|---------|--------|--------| | Transport | TCP | TCP | QUIC (UDP-based) | | Head-of-line blocking | Yes | At TCP level | No | | Multiplexing | No (pipelining limited) | Yes | Yes | | Header compression | No | Yes (HPACK) | Yes (QPACK) | | Server push | No | Yes | Yes | | Connection reuse | Keep-alive | Full multiplexing | Full multiplexing | | Adoption | Legacy | Modern (2020+) | Cutting edge (2023+) |

DNS (Domain Name System)

DNS translates human-readable domain names into IP addresses. The resolution process is hierarchical:

Browser → Local Cache → Resolver → Root (.) → TLD (.com) → Authoritative (example.com)

DNS Record Types

| Type | Purpose | Example | |------|---------|--------| | A | IPv4 address | example.com → 93.184.216.34 | | AAAA | IPv6 address | example.com → 2606:2800:220:1:248:1893:25c8:1946 | | CNAME | Canonical name (alias) | www.example.com → example.com | | MX | Mail exchange | @ → mail.example.com | | TXT | Arbitrary text (verification, SPF) | v=spf1 include:_spf.google.com | | NS | Nameserver | example.com → ns1.example.com | | SOA | Start of authority | Zone transfer parameters |

TLS/SSL

TLS (Transport Layer Security) encrypts data between client and server.

TLS 1.3 Handshake

Client                     Server
  |    ClientHello (key_share) |
  |------------------------->|
  |    ServerHello + Certificate + Finished |
  |<-------------------------|
  |    Finished               |
  |------------------------->|
  |     Encrypted Data         |
  |<=========================>|

TLS 1.3 requires only one round trip (1-RTT) vs. TLS 1.2 which requires 2-RTT. TLS 1.3 also removes insecure cipher suites and provides forward secrecy by default.

HTTPS Flow Summary

TCP Handshake (1 RTT) + TLS Handshake (1 RTT with TLS 1.3) → First byte of HTTP request at 2 RTT. With HTTP/3 over QUIC, the connection is established in 0-RTT for repeat visitors.

Common Network Debugging Commands

| Command | What It Does | |---------|-------------| | ping example.com | Check reachability and round-trip time | | traceroute example.com | Show each hop from client to server | | nslookup example.com | Query DNS records | | dig example.com | Detailed DNS query | | curl -v https://example.com | Full HTTP request/response debug | | tcpdump -i eth0 port 443 | Capture network packets | | ss -tuln | List listening ports | | netstat -an | Network statistics (older) |

Summary

Networking is the backbone of modern computing. Understanding TCP/IP, DNS, HTTP, and TLS helps you build reliable web applications, debug production issues, and design scalable systems. Every web developer should know what happens when they type a URL in their browser.

Key takeaways:

DNS resolves domain names to IP addresses through a hierarchical lookup
TCP provides reliable, ordered connections via a three-way handshake
UDP is faster but unreliable — used for streaming, DNS, gaming
HTTP/2 and HTTP/3 improve performance over HTTP/1.1 with multiplexing
TLS encrypts HTTP traffic — TLS 1.3 reduces handshake to 1 RTT
HTTPS = HTTP over TLS (port 443)
When type a URL: DNS → TCP → TLS → HTTP request → server processes → HTTP response → browser renders

What's Next: Data Structures

The next chapter covers fundamental data structures — arrays, linked lists, trees, hash tables, and when to use each.

HTTP Headers Reference

Request Headers

| Header | Purpose | Example | |--------|---------|--------| | Host | Target domain | Host: example.com | | User-Agent | Client identification | User-Agent: Mozilla/5.0... | | Accept | Expected response format | Accept: application/json | | Authorization | Authentication credentials | Authorization: Bearer token | | Content-Type | Body format | Content-Type: application/json | | Referer | Previous page URL | Referer: https://google.com | | Cookie | Stored cookies | Cookie: session=abc123 | | Cache-Control | Caching directives | Cache-Control: no-cache |

Response Headers

| Header | Purpose | Example | |--------|---------|--------| | Content-Type | Response body format | Content-Type: text/html; charset=utf-8 | | Content-Length | Body size in bytes | Content-Length: 1234 | | Set-Cookie | Set a cookie | Set-Cookie: session=abc123; HttpOnly | | Cache-Control | Caching instructions | Cache-Control: max-age=3600 | | Location | Redirect target | Location: /new-page | | Access-Control-Allow-Origin | CORS policy | Access-Control-Allow-Origin: * |

Understanding HTTP headers is essential for debugging API issues and configuring web servers correctly.