CIDR notation in Computer Networks - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding CIDR Prefix Length

What does the prefix length in CIDR notation represent?

A The number of bits used for the network portion of the address

B The total number of bits in the IP address

C The number of hosts allowed in the network

D The number of bits used for the host portion of the address

💡 Hint

Think about how CIDR divides the IP address into network and host parts.

The prefix length specifies how many bits from the start of the IP address are used to identify the network. The remaining bits are for hosts.

📋 Factual

intermediate

CIDR Notation Range Calculation

Given the CIDR notation 192.168.1.0/26, how many usable IP addresses are available in this subnet?

A 64

B 62

C 32

D 30

💡 Hint

Calculate 2 to the power of host bits, then subtract network and broadcast addresses.

/26 means 32 - 26 = 6 bits for hosts. 2^6 = 64 addresses total. Subtract 2 for network and broadcast, so 62 usable.

🔍 Analysis

advanced

CIDR Block Overlap Detection

Which of the following CIDR blocks overlaps with 10.0.0.0/16?

A 192.168.0.0/16

B 10.1.0.0/24

C 10.0.128.0/17

D 10.0.0.0/24

💡 Hint

Check if the IP ranges covered by the blocks share any addresses.

10.0.0.0/16 covers 10.0.0.0 to 10.0.255.255. 10.0.128.0/17 covers 10.0.128.0 to 10.0.255.255, so they overlap.

❓ Comparison

advanced

Comparing CIDR Prefixes

Which CIDR prefix represents the smallest subnet?

A 172.16.0.0/16

B 172.16.0.0/20

C 172.16.0.0/18

D 172.16.0.0/22

💡 Hint

Smaller subnet means fewer host bits, so larger prefix number.

/22 has more bits for network than /20, /18, or /16, so it covers fewer addresses and is the smallest subnet.

❓ Reasoning

expert

CIDR Aggregation Possibility

Which pair of CIDR blocks can be aggregated into a single larger CIDR block without including extra IP addresses?

A 192.168.0.0/25 and 192.168.0.128/25

B 192.168.0.0/24 and 192.168.1.0/24

C 10.0.0.0/24 and 10.0.1.0/25

D 172.16.0.0/23 and 172.16.2.0/23

💡 Hint

Aggregation requires contiguous blocks with matching prefix lengths and aligned boundaries.

192.168.0.0/25 covers 0-127, 192.168.0.128/25 covers 128-255. Together they form 192.168.0.0/24 exactly.

Practice

1. What does the number after the slash in a CIDR notation represent? For example, in 192.168.1.0/24.

easy

A. The total number of IP addresses in the network

B. The size of the network or how many bits are used for the network part

C. The number of hosts allowed in the network

D. The version of the IP address

Solution

1. Step 1: Understand CIDR notation structure

   CIDR notation shows an IP address followed by a slash and a number. This number tells how many bits are used for the network part of the address.

2. Step 2: Interpret the number after the slash

   The number after the slash (like 24 in 192.168.1.0/24) means the first 24 bits are the network portion, defining the network size.

3. Final Answer:

   The size of the network or how many bits are used for the network part -> Option B

4. Quick Check:

   Number after slash = network bits [OK]

Hint: Slash number shows network bits, not hosts or total IPs [OK]

Common Mistakes:

• Confusing slash number with total hosts
• Thinking slash number is IP version
• Assuming slash number shows number of hosts

2. Which of the following is the correct CIDR notation for a network with IP address 10.0.0.0 and a subnet mask of 255.255.255.0?

easy

A. 10.0.0.0/16

B. 10.0.0.0/8

C. 10.0.0.0/24

D. 10.0.0.0/32

Solution

1. Step 1: Convert subnet mask to CIDR prefix length

   Subnet mask 255.255.255.0 means the first 24 bits are ones (11111111.11111111.11111111.00000000), so the CIDR prefix is /24.

2. Step 2: Match IP with correct CIDR notation

   Given IP 10.0.0.0 with subnet mask 255.255.255.0 corresponds to 10.0.0.0/24.

3. Final Answer:

   10.0.0.0/24 -> Option C

4. Quick Check:

   255.255.255.0 = /24 [OK]

Hint: Count 1 bits in subnet mask to find slash number [OK]

Common Mistakes:

• Confusing /16 with 255.255.0.0 mask
• Using /32 which means single IP
• Mixing up subnet mask and CIDR prefix

3. Given the CIDR notation 192.168.10.0/28, how many usable IP addresses are available in this subnet?

medium

A. 14

B. 28

C. 30

D. 16

Solution

1. Step 1: Calculate total IP addresses from CIDR prefix

   /28 means 28 bits for network, so 32 - 28 = 4 bits for hosts. Total IPs = 2^4 = 16.

2. Step 2: Calculate usable IP addresses

   Usable IPs exclude network and broadcast addresses, so usable = 16 - 2 = 14.

3. Final Answer:

   14 -> Option A

4. Quick Check:

   Usable IPs = 2^(32-28) - 2 = 14 [OK]

Hint: Usable IPs = 2^(32 - slash) minus 2 [OK]

Common Mistakes:

• Counting total IPs as usable
• Forgetting to subtract network and broadcast addresses
• Miscalculating bits for hosts

4. A network administrator wrote the CIDR notation as 172.16.0.0/33. What is wrong with this notation?

medium

A. CIDR prefix must be less than 16

B. IP address is invalid

C. Subnet mask is missing

D. CIDR prefix cannot be greater than 32 for IPv4

Solution

1. Step 1: Understand CIDR prefix limits for IPv4

   IPv4 addresses are 32 bits long, so CIDR prefix must be between 0 and 32 inclusive.

2. Step 2: Identify invalid prefix

   A prefix of 33 is invalid because it exceeds the 32-bit length of IPv4 addresses.

3. Final Answer:

   CIDR prefix cannot be greater than 32 for IPv4 -> Option D

4. Quick Check:

   IPv4 prefix max = 32 [OK]

Hint: CIDR prefix max 32 for IPv4, 128 for IPv6 [OK]

Common Mistakes:

• Thinking IP address is invalid instead
• Confusing CIDR prefix with subnet mask
• Assuming prefix must be less than 16

5. You need to create a subnet that supports at least 1000 hosts. Which CIDR notation would be the smallest network that fits this requirement?

hard

A. 192.168.0.0/22

B. 192.168.0.0/23

C. 192.168.0.0/21

D. 192.168.0.0/24

Solution

1. Step 1: Calculate required host bits for 1000 hosts

   Hosts = 2^(host bits) - 2. We want at least 1000 usable hosts. Try host bits: 10 bits gives 2^10 - 2 = 1022 hosts, which is enough.

2. Step 2: Calculate CIDR prefix from host bits

   IPv4 has 32 bits total. Network bits = 32 - host bits = 32 - 10 = 22. So /22 supports 1022 hosts.

3. Step 3: Check options for smallest network

   /22 supports 1022 hosts, /23 supports 510 hosts (too small), /21 supports 2046 hosts (larger than needed), /24 supports 254 hosts (too small). Smallest that fits is /22.

4. Final Answer:

   192.168.0.0/22 -> Option A

5. Quick Check:

   Hosts ≥ 1000 -> /22 (1022 hosts) [OK]

Hint: Use 2^(32 - prefix) - 2 ≥ hosts needed [OK]

Common Mistakes:

• Choosing /23 or /24 which are too small
• Not subtracting 2 for network and broadcast
• Picking /21 which is larger than needed