DNS (Domain Name System) in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

How DNS Resolves a Domain Name

When you type a website address like www.example.com in your browser, how does DNS help your computer find the website?

Choose the best description of the DNS resolution process.

A DNS translates the domain name into an IP address by querying a series of DNS servers starting from the root, then top-level domain, and finally the authoritative server.

B DNS directly connects to the website server without any translation, using the domain name as the address.

C DNS stores the website content locally and sends it to your browser when you enter the domain name.

D DNS encrypts your domain name and sends it to the internet service provider to find the website.

💡 Hint

Think about how computers use numbers (IP addresses) to find each other, not names.

DNS works by translating human-friendly domain names into IP addresses computers use. It does this by querying DNS servers in a hierarchical order: root servers, then top-level domain servers, and finally authoritative servers for the domain.

📋 Factual

intermediate

Purpose of a DNS Cache

What is the main purpose of a DNS cache on your computer or network?

A To block access to certain websites by storing their IP addresses.

B To store IP addresses of recently visited domain names to speed up future lookups.

C To permanently save all domain names and their IP addresses for offline use.

D To encrypt DNS queries to protect user privacy.

💡 Hint

Think about how remembering something recently used can make things faster.

A DNS cache temporarily stores IP addresses of domain names recently looked up. This reduces the time needed for future requests to the same domain by avoiding repeated queries to DNS servers.

🔍 Analysis

advanced

Identifying DNS Record Types

Given the following DNS record, what type of record is it and what does it do?

example.com. 3600 IN MX 10 mail.example.com.

A A record; it maps a domain to an IPv4 address.

B CNAME record; it aliases one domain name to another.

C MX record; it specifies the mail server responsible for receiving email for the domain.

D TXT record; it stores text information for the domain.

💡 Hint

Look at the record type abbreviation and think about email.

An MX (Mail Exchange) record tells mail servers where to deliver email for the domain. The number 10 is the priority of the mail server.

❓ Comparison

advanced

Difference Between Recursive and Iterative DNS Queries

Which statement correctly describes the difference between recursive and iterative DNS queries?

A In a recursive query, the DNS server fully resolves the domain name and returns the final IP address; in an iterative query, the server returns the best answer it has or a referral to another server.

B In a recursive query, the client resolves the domain name by itself; in an iterative query, the server resolves it fully for the client.

C Recursive queries are used only for email servers; iterative queries are used for websites.

D Recursive queries always return multiple IP addresses; iterative queries return only one.

💡 Hint

Consider who does the work of finding the final answer in each query type.

Recursive queries require the DNS server to find the full answer and return it. Iterative queries return the best information the server has or a referral to another server for the client to query next.

❓ Reasoning

expert

Impact of DNS TTL on Website Accessibility

If a website's DNS record has a very high TTL (Time To Live) value, what could be a potential downside when the website's IP address changes?

A High TTL values cause DNS queries to fail more often.

B The website will immediately update for all users, causing no downtime.

C The DNS servers will ignore the TTL and update instantly to the new IP address.

D Users may continue to be directed to the old IP address for a long time, causing access problems until caches expire.

💡 Hint

Think about how caching works and what happens when information changes.

A high TTL means DNS records are cached longer. If the IP changes, users with cached old records may not reach the new IP until the cache expires, causing delays in updates and possible downtime.

Practice

1. What is the main purpose of the DNS in computer networks?

easy

A. To store website content

B. To translate website names into IP addresses

C. To secure internet connections

D. To speed up internet downloads

Solution

1. Step 1: Understand what DNS stands for

   DNS means Domain Name System, which helps computers find websites.

2. Step 2: Identify DNS function

   DNS translates easy-to-remember website names (like example.com) into IP addresses that computers use to connect.

3. Final Answer:

   To translate website names into IP addresses -> Option B

4. Quick Check:

   DNS translates names to IPs = A [OK]

Hint: DNS matches names to numbers (IP addresses) [OK]

Common Mistakes:

• Thinking DNS stores website content
• Confusing DNS with security protocols
• Believing DNS speeds up downloads

2. Which of the following is the correct format of a DNS record that maps a domain to an IP address?

easy

A. TXT record

B. MX record

C. CNAME record

D. A record

Solution

1. Step 1: Recall DNS record types

   DNS uses different record types for different purposes: A records map domains to IP addresses, MX records handle mail servers, TXT records store text info, and CNAME records alias domains.

2. Step 2: Identify record for IP mapping

   The A record specifically maps a domain name to an IPv4 address.

3. Final Answer:

   A record -> Option D

4. Quick Check:

   A record = IP mapping [OK]

Hint: A record = Address (IP) mapping [OK]

Common Mistakes:

• Confusing MX with IP mapping
• Thinking TXT stores IP addresses
• Mixing CNAME with direct IP mapping

3. If you type www.example.com in your browser, which sequence correctly describes what DNS does?

medium

A. Browser sends IP to DNS server -> DNS returns IP -> Browser connects to IP

B. Browser sends IP to DNS server -> DNS returns domain -> Browser connects to domain

C. Browser sends domain to DNS server -> DNS returns IP -> Browser connects to IP

D. Browser sends domain to DNS server -> DNS returns website content -> Browser displays content

Solution

1. Step 1: Understand DNS query process

   When you enter a domain, the browser asks DNS servers for the IP address of that domain.

2. Step 2: Follow DNS response and connection

   The DNS server replies with the IP address, then the browser uses that IP to connect to the website.

3. Final Answer:

   Browser sends domain to DNS server -> DNS returns IP -> Browser connects to IP -> Option C

4. Quick Check:

   Domain to IP to connect = D [OK]

Hint: DNS translates domain to IP before connecting [OK]

Common Mistakes:

• Thinking DNS returns website content
• Confusing domain and IP roles
• Assuming DNS returns domain from IP

4. A user cannot access a website even though the internet connection is fine. Which DNS-related issue is most likely the cause?

medium

A. Incorrect IP address in DNS record

B. Slow internet speed

C. Browser cache is full

D. Website server is down

Solution

1. Step 1: Analyze the problem context

   The internet connection works, so the problem is not network speed or browser cache.

2. Step 2: Identify DNS issue

   If DNS has an incorrect IP address for the website, the browser cannot find the correct server, causing access failure.

3. Final Answer:

   Incorrect IP address in DNS record -> Option A

4. Quick Check:

   Wrong DNS IP causes access failure = B [OK]

Hint: Wrong IP in DNS stops website loading [OK]

Common Mistakes:

• Blaming slow internet when connection is fine
• Assuming browser cache causes DNS errors
• Confusing server downtime with DNS issues

5. You want to create a DNS entry that points blog.example.com to example.com without assigning a new IP. Which DNS record type should you use?

hard

A. CNAME record

B. A record

C. MX record

D. PTR record

Solution

1. Step 1: Understand DNS record purposes

   A records map domains to IPs, CNAME records alias one domain to another, MX records handle mail, PTR records map IPs to domains.

2. Step 2: Choose record for aliasing

   To point blog.example.com to example.com without a new IP, a CNAME record creates an alias to the main domain.

3. Final Answer:

   CNAME record -> Option A

4. Quick Check:

   Alias domain uses CNAME record = A [OK]

Hint: Use CNAME to alias one domain to another [OK]

Common Mistakes:

• Using A record for alias instead of IP
• Confusing MX with domain aliasing
• Thinking PTR is for forward DNS