Networking Basics: OSI Model, TCP/IP, DNS, and HTTP

What Happens When You Type a URL?

Before diving into abstract models and protocols, let's ground ourselves in something concrete. Every time you type https://google.com into your browser, a complex choreography of systems kicks into action. Understanding this end-to-end flow is the single most valuable mental model in system design.

Here's what happens, simplified:

1. DNS Lookup — Your browser asks "what IP is google.com?"
2. TCP Connection — Your computer establishes a connection to that IP
3. TLS Handshake — The connection is encrypted (HTTPS)
4. HTTP Request — Your browser asks for the webpage
5. Server Response — The server sends back HTML, CSS, JavaScript
6. Browser Rendering — Your browser paints the page

Each of these six steps involves different layers of the networking stack. Let's understand each one.

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The OSI Model (7 Layers)

The OSI (Open Systems Interconnection) model is a conceptual framework that describes how data flows from a software application on one computer to a software application on another. It divides networking into 7 layers, each with a specific responsibility.

How the Layers Work Together

When you send an HTTP request, data flows down the layers on your machine, across the network, and up the layers on the server:

Each layer encapsulates the data from the layer above it, adding its own header. This is why understanding the layers matters — when you debug a slow API, you need to know whether the problem is at Layer 4 (TCP retransmissions), Layer 3 (routing), or Layer 7 (slow application logic).

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TCP vs UDP

The Transport Layer (Layer 4) has two main protocols, and choosing between them is one of the most fundamental design decisions you'll make.

TCP (Transmission Control Protocol)

TCP is reliable, ordered, and connection-oriented. It guarantees that:

• All data arrives
• Data arrives in order
• Corrupted data is retransmitted

It achieves this through the TCP 3-way handshake:

When to use TCP: Web browsing (HTTP), email (SMTP), file transfer (FTP), database connections — anywhere data loss is unacceptable.

UDP (User Datagram Protocol)

UDP is unreliable, unordered, and connectionless. It just sends packets (datagrams) and hopes they arrive. No handshake, no acknowledgment, no retransmission.

When to use UDP: Video streaming, VoIP, DNS queries, online gaming, real-time sensor data — where speed matters more than perfect delivery.

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DNS — The Internet's Phone Book

DNS (Domain Name System) translates human-readable domain names (google.com) into machine-readable IP addresses (142.250.80.46). Without DNS, you'd need to memorize IP addresses for every website.

DNS Resolution Steps

When you type google.com, here's the lookup chain:

1. Browser Cache — Does your browser already know this IP?
2. OS Cache — Does your operating system's DNS cache have it?
3. Router Cache — Does your home router know?
4. ISP DNS Resolver — Your ISP's recursive resolver checks its cache
5. Root Server — "I don't know google.com, but here's who handles .com"
6. TLD Server — "I don't know google.com, but here's Google's authoritative DNS"
7. Authoritative Server — "google.com → 142.250.80.46"

DNS Record Types

DNS and System Design

In system design, DNS matters because:

• Load balancing — DNS round-robin distributes traffic across multiple IPs
• Geo-DNS — Return different IPs based on the user's location (Cloudflare, AWS Route 53)
• CDN routing — Direct users to the nearest edge server
• Failover — DNS-based failover to a backup region

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HTTP/1.1 vs HTTP/2 vs HTTP/3

HTTP is the Application Layer protocol (Layer 7) that powers the web. It has evolved significantly.

HTTP/1.1 (1997)

• Text-based protocol
• One request per TCP connection (head-of-line blocking at the application layer)
• Workarounds: multiple connections (6 per domain), domain sharding, sprites, inlining
• Persistent connections (keep-alive) — reuse TCP connection for multiple requests

HTTP/2 (2015)

• Binary protocol (not text)
• Multiplexing — multiple requests/responses over a single TCP connection
• Header compression (HPACK) — reduces overhead
• Server push — server can send resources the client hasn't requested yet
• Solves HTTP/1.1's head-of-line blocking, but still blocked by TCP head-of-line blocking (if one TCP packet is lost, all streams wait)

HTTP/3 (2022)

• Uses QUIC (Quick UDP Internet Connections) instead of TCP
• Built on UDP — eliminates TCP head-of-line blocking
• 0-RTT connection establishment — faster than TCP's 3-way handshake + TLS handshake
• Connection migration — switch networks (WiFi → cellular) without reconnecting

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What to Remember for Interviews

1. OSI Model — Know all 7 layers and one protocol per layer
2. TCP 3-way handshake — Draw it, explain it, know why it exists (prevent stale connections, synchronize sequence numbers)
3. DNS resolution chain — Browser → OS → ISP → Root → TLD → Authoritative
4. HTTP evolution — HTTP/1.1 (sequential) → HTTP/2 (multiplexed over TCP) → HTTP/3 (QUIC over UDP)
5. TCP vs UDP trade-offs — Reliability vs latency; know when to use each