CIDR notation in Computer Networks - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to represent the CIDR notation for the IP address 192.168.1.0 with a subnet mask of 24 bits.

Computer Networks

cidr = "192.168.1.0/[1]"

Drag options to blanks, or click blank then click option'

A16

B8

C32

D24

Attempts:

3 left

💡 Hint

Common Mistakes

Using the subnet mask in decimal form instead of the number of bits.

Confusing the number of bits with the number of octets.

The CIDR notation uses a slash followed by the number of bits in the subnet mask. For a common subnet mask of 255.255.255.0, the number of bits is 24.

2fill in blank

medium

Complete the code to calculate the number of possible hosts in a subnet given the CIDR prefix length.

Computer Networks

hosts = 2**(32 - [1]) - 2

Drag options to blanks, or click blank then click option'

A24

B16

C28

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to subtract 2 for network and broadcast addresses.

Using the prefix length directly instead of subtracting from 32.

The number of hosts is calculated by subtracting the prefix length from 32 (total bits in IPv4), then raising 2 to that power and subtracting 2 for network and broadcast addresses.

3fill in blank

hard

Fix the error in the code that converts a CIDR prefix length to a subnet mask in dotted decimal format.

Computer Networks

subnet_mask = '.'.join(str((0xffffffff << (32 - [1]) >> i) & 0xff) for i in [24,16,8,0])

Drag options to blanks, or click blank then click option'

Amask_length

Bcidr

Cprefix_length

Dsubnet_bits

Attempts:

3 left

💡 Hint

Common Mistakes

Using undefined variable names.

Mixing variable names that represent different concepts.

The variable representing the CIDR prefix length should be named consistently. Here, 'prefix_length' is the correct variable to use.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that maps CIDR prefixes to their subnet masks in dotted decimal format for prefixes 24 and 16.

Computer Networks

cidr_to_mask = {prefix: '.'.join(str((0xffffffff << (32 - prefix) >> i) & 0xff) for i in [1]) for prefix in [2]

Drag options to blanks, or click blank then click option'

A[24, 16, 8, 0]

B[16, 24]

C[0, 8, 16, 24]

D[8, 16]

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up the order of bit shifts.

Using incorrect prefix lists.

The bit shifts for each octet are 24, 16, 8, and 0. The prefixes to map are 24 and 16.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps CIDR prefixes to the number of hosts available in each subnet for prefixes 28, 26, and 24.

Computer Networks

hosts_per_subnet = {prefix: 2**(32 - [1]) - [2] for prefix in [3]

Drag options to blanks, or click blank then click option'

Aprefix

B2

C[28, 26, 24]

D32

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable name for the prefix.

Forgetting to subtract 2 from the total hosts.

Using incorrect prefix lists.

The variable 'prefix' is used for each CIDR prefix. The number 2 is subtracted to exclude network and broadcast addresses. The prefixes are 28, 26, and 24.

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