Subnetting and subnet masks in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding subnet mask purpose

What is the primary purpose of a subnet mask in a network?

A To divide an IP address into network and host parts

B To assign IP addresses to devices automatically

C To encrypt data sent over the network

D To increase the speed of data transmission

💡 Hint

Think about how networks separate devices into groups.

A subnet mask helps identify which part of an IP address refers to the network and which part refers to the host (device). This division allows networks to be split into smaller subnetworks.

📋 Factual

intermediate

Number of hosts in a subnet

If a subnet mask is 255.255.255.240, how many usable host addresses are available in that subnet?

A 32

B 16

C 14

D 30

💡 Hint

Calculate the number of host bits and subtract network and broadcast addresses.

The mask 255.255.255.240 corresponds to 28 bits for the network and 4 bits for hosts. 2^4 = 16 addresses total, but 2 are reserved (network and broadcast), so 14 usable hosts.

🔍 Analysis

advanced

Determining subnet mask from hosts needed

You need to create a subnet that supports at least 100 hosts. Which subnet mask should you use?

A 255.255.255.128

B 255.255.255.192

C 255.255.255.0

D 255.255.255.224

💡 Hint

Calculate how many host bits are needed to support 100 hosts.

255.255.255.128 has 7 host bits (2^7=128 addresses). Subtracting 2 reserved addresses gives 126 usable hosts, enough for 100 hosts. Other masks provide fewer usable hosts.

❓ Comparison

advanced

Comparing subnet masks and network sizes

Which subnet mask creates the largest number of subnets but the smallest number of hosts per subnet?

A 255.255.255.0

B 255.255.255.128

C 255.255.255.192

D 255.255.255.224

💡 Hint

More subnet bits mean more subnets but fewer hosts per subnet.

255.255.255.224 uses 3 bits for subnetting (more subnets) and 5 bits for hosts (fewer hosts). It creates more subnets but fewer hosts per subnet compared to others.

❓ Reasoning

expert

Identifying subnet mask from IP and broadcast address

You have a subnet with IP address 192.168.1.64 and broadcast address 192.168.1.95. What is the subnet mask?

A 255.255.255.192

B 255.255.255.224

C 255.255.255.240

D 255.255.255.128

💡 Hint

Calculate the range size between IP and broadcast to find host bits.

The range from 64 to 95 is 32 addresses. 2^5=32, so 5 host bits. The mask with 5 host bits is 255.255.255.224.

Practice

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

easy

A. To connect different types of devices without IP addresses

B. To increase the total number of IP addresses available globally

C. To encrypt data sent over the network

D. To divide a large network into smaller, manageable parts

Solution

1. Step 1: Understand subnetting concept

   Subnetting is used to split a large network into smaller subnetworks for easier management.

2. Step 2: Compare options

   Only To divide a large network into smaller, manageable parts correctly describes subnetting's purpose; others describe unrelated functions.

3. Final Answer:

   To divide a large network into smaller, manageable parts -> Option D

4. Quick Check:

   Subnetting = dividing networks [OK]

Hint: Subnetting means splitting networks into smaller parts [OK]

Common Mistakes:

• Confusing subnetting with encryption
• Thinking subnetting increases global IP addresses
• Believing subnetting connects devices without IPs

2. Which of the following is a valid subnet mask?

easy

A. 255.255.255.0

B. 0.255.255.255

C. 255.0.255.0

D. 255.255.0.255

Solution

1. Step 1: Recall valid subnet mask format

   A subnet mask must have continuous 1s followed by continuous 0s in binary, like 255.255.255.0.

2. Step 2: Check each option

   Only 255.255.255.0 fits this rule; others have mixed or invalid patterns.

3. Final Answer:

   255.255.255.0 -> Option A

4. Quick Check:

   Valid subnet mask = continuous 1s then 0s [OK]

Hint: Subnet masks have continuous 255s then 0s [OK]

Common Mistakes:

• Choosing masks with mixed 255 and 0 in wrong order
• Not knowing binary pattern rules for subnet masks
• Confusing IP addresses with subnet masks

3. Given the IP address 192.168.1.10 and subnet mask 255.255.255.0, what is the network address?

medium

A. 192.168.1.0

B. 192.168.0.0

C. 192.168.1.255

D. 192.168.0.255

Solution

1. Step 1: Understand network address calculation

   The network address is found by performing a bitwise AND between the IP address and subnet mask.

2. Step 2: Calculate network address

   IP: 192.168.1.10 AND Mask: 255.255.255.0 gives 192.168.1.0 as the network address.

3. Final Answer:

   192.168.1.0 -> Option A

4. Quick Check:

   Network address = IP AND subnet mask [OK]

Hint: AND IP with mask to get network address [OK]

Common Mistakes:

• Confusing broadcast address with network address
• Using wrong subnet mask for calculation
• Not applying bitwise AND operation

4. Identify the error in this subnet mask: 255.255.224.255

medium

A. Subnet mask must always end with 0 in the last octet

B. Subnet mask cannot have 224 in any octet

C. Subnet mask must have continuous 1s followed by 0s; this mask breaks that rule

D. Subnet mask cannot have more than two 255 octets

Solution

1. Step 1: Check binary pattern of subnet mask

   Subnet masks must have all 1s on the left side and all 0s on the right side without interruption.

2. Step 2: Analyze given mask

   255.255.224.255 breaks this rule because the last octet is 255 (all 1s) after 224 (which is partial 1s), causing non-continuous 1s.

3. Final Answer:

   Subnet mask must have continuous 1s followed by 0s; this mask breaks that rule -> Option C

4. Quick Check:

   Subnet mask = continuous 1s then 0s [OK]

Hint: Subnet mask 1s must be continuous, no breaks [OK]

Common Mistakes:

• Thinking 224 is invalid in subnet masks
• Believing last octet must always be 0
• Ignoring binary continuity rule

5. You have a network 10.0.0.0/24 and need at least 6 subnets. Which subnet mask should you use to create enough subnets?

hard

A. 255.255.255.192

B. 255.255.255.224

C. 255.255.255.0

D. 255.255.255.128

Solution

1. Step 1: Calculate required subnet bits

   To create at least 6 subnets, find minimum bits n where 2^n ≥ 6. Here, n=3 (2^3=8 subnets).

2. Step 2: Determine new subnet mask

   Original mask /24 means 24 bits for network. Adding 3 bits for subnetting gives /27 mask.

3. Step 3: Convert /27 to subnet mask

   /27 corresponds to 255.255.255.224.

4. Final Answer:

   255.255.255.224 -> Option B

5. Quick Check:

   3 subnet bits = /27 = 255.255.255.224 [OK]

Hint: Use 2^n ≥ needed subnets to find mask [OK]

Common Mistakes:

• Choosing mask with too few subnet bits
• Confusing host bits with subnet bits
• Not converting CIDR to decimal mask correctly