DNS (Domain Name System)

🤔 What is DNS? (Basic Idea)

Humans prefer names.

Computers understand numbers.

📌 Example:

• Human-friendly name 👉 www.google.com

• Computer-friendly address 👉 142.250.195.46

👉 DNS acts as a translator between names and numbers.

🧠 Simple Definition

📌 DNS (Domain Name System) is a distributed naming system that translates human-readable domain names into IP addresses.

🧠 Why DNS is Needed

Imagine if you had to remember IP addresses like:

❌ 142.250.195.46

❌ 216.58.203.110

Instead of:

✔️ google.com

✔️ youtube.com

👉 That would be impossible 😵

📌 DNS solves this problem by:

• Allowing users to use easy names

• Converting them into IP addresses

🧠 Real-Life Analogy

📞 Phone Contact List

• You save a name → “Mom”

• Phone internally stores the number

👉 DNS works the same way:

• You type domain name

• DNS finds the IP address

🧭 Where Does DNS Belong in OSI Model?

• DNS is an Application Layer protocol

• Uses:

  • UDP (port 53) mostly

  • TCP (port 53) for large responses

📦 What Does DNS Actually Do?

DNS performs:

• 🌐 Name → IP translation

• 🔁 IP → Name translation (reverse DNS)

• 📬 Mail server lookup

• 🧭 Load distribution

👉 DNS is much more than just name lookup.

🧱 Structure of DNS (Hierarchical System)

DNS follows a hierarchical tree structure:

Root(.)
 └──Top-LevelDomain(TLD)
      └──Second-LevelDomain
           └──Subdomain

🧩 Example: www.google.com

🌍 Types of DNS Domains

🔹 Top-Level Domains (TLDs)

Examples:

• .com – Commercial

• .org – Organization

• .edu – Education

• .gov – Government

• .in – India

🔹 Second-Level Domains

• Registered by organizations

• Example: google, amazon

🔹 Subdomains

• Created by organizations

• Example: mail.google.com

🧠 DNS Server Types

DNS is distributed, not centralized.

🌐 1. Root DNS Server

• Top of DNS hierarchy

• Knows where TLD servers are

📌 There are 13 logical root servers worldwide.

🌍 2. TLD DNS Server

• Manages domains like .com, .org, .in

• Directs to authoritative servers

🏢 3. Authoritative DNS Server

• Stores actual IP address of domain

• Final answer provider

📌 Example:

• Google’s DNS server knows IP of google.com

💻 4. Local DNS Server (Resolver)

• Provided by ISP

• First DNS contacted by your computer

📌 Acts as a middleman.

🔄 How DNS Works (Step-by-Step)

Let’s say you type:

👉 www.google.com

🪜 Step-by-Step DNS Resolution

1️⃣ Browser checks cache

2️⃣ Query sent to Local DNS Server

3️⃣ Local DNS asks Root Server

4️⃣ Root replies with TLD server info

5️⃣ Local DNS asks TLD server

6️⃣ TLD replies with Authoritative server info

7️⃣ Local DNS asks Authoritative server

8️⃣ IP address returned

9️⃣ Browser connects to server using IP

📌 All this happens in milliseconds ⚡

🧠 DNS Caching (Performance Booster)

📌 DNS uses caching to:

• Reduce delay

• Reduce load on servers

Types of Cache:

• Browser cache

• OS cache

• Local DNS cache

👉 Cached entries have TTL (Time To Live).

🔁 Types of DNS Queries

🔹 Recursive Query

• Client wants final answer

• DNS server does all the work

📌 Most common type.

🔹 Iterative Query

• DNS server gives partial answer

• Client continues querying

📌 Used between DNS servers.

📜 Important DNS Record Types

🧠 Example DNS Records

google.comA142.250.195.46
google.com   MX    mail.google.com
www.google.com CNAME google.com

⚡ DNS over UDP vs TCP

📌 Default DNS port: 53

🔐 DNS Security Issues (Basic Idea)

• DNS spoofing

• DNS cache poisoning

📌 Modern solution:

• DNSSEC (adds security)

✍️ Definitions

📌 DNS:

DNS is a distributed naming system that translates domain names into IP addresses.

📌 Authoritative DNS Server:

A DNS server that stores the original IP address for a domain.

🧠 Memory Tricks

• DNS = Internet Phonebook

• A = Address record

• MX = Mail exchange

⭐ One-Line Summary

🔹 DNS translates human-readable domain names into IP addresses, enabling users to access Internet services easily.