CIDR notation in Computer Networks - Mini Project: Build & Apply

Understanding CIDR Notation

📖 Scenario: You are working as a network technician setting up IP addresses for a small office network. You need to understand how to write and interpret IP addresses using CIDR notation to efficiently allocate IP ranges.

🎯 Goal: Build a simple reference guide that shows how to write an IP address with its subnet mask in CIDR notation, and how to identify the network and host parts.

📋 What You'll Learn

Create a dictionary called ip_addresses with three IP addresses as keys and their subnet masks as values.

Add a variable called cidr_suffix that holds the number of bits used for the network part.

Create a new dictionary called cidr_notation that combines each IP address with the cidr_suffix to form CIDR notation strings.

Add a final dictionary entry that explains what CIDR notation means in simple terms.

💡 Why This Matters

🌍 Real World

CIDR notation is used by network engineers and IT professionals to efficiently allocate IP addresses and manage networks.

💼 Career

Understanding CIDR notation is essential for roles in network administration, cybersecurity, and cloud infrastructure management.

1

Create the IP addresses dictionary

Create a dictionary called ip_addresses with these exact entries: '192.168.1.0' with subnet mask '255.255.255.0', '10.0.0.0' with subnet mask '255.0.0.0', and '172.16.0.0' with subnet mask '255.240.0.0'.

Computer Networks

# Create the ip_addresses dictionary with given IPs and subnet masks
# Your code here

Hint

Use curly braces {} to create a dictionary with the IP addresses as keys and subnet masks as values.

2

Add the CIDR suffix variable

Add a variable called cidr_suffix and set it to 24 to represent the number of bits used for the network part in CIDR notation.

Computer Networks

ip_addresses = {
    '192.168.1.0': '255.255.255.0',
    '10.0.0.0': '255.0.0.0',
    '172.16.0.0': '255.240.0.0'
}
# Add the cidr_suffix variable below
# Your code here

Hint

Simply assign the number 24 to the variable cidr_suffix.

3

Create the CIDR notation dictionary

Create a new dictionary called cidr_notation that uses a for loop with variables ip and mask to iterate over ip_addresses.items(). For each IP, combine it with cidr_suffix to form a string like '192.168.1.0/24' and store it as the value in cidr_notation with the IP as the key.

Computer Networks

cidr_notation = {}
for ip, mask in ip_addresses.items():
    cidr_notation[ip] = f"{ip}/{cidr_suffix}"

Hint

Use a for loop to build the new dictionary by combining the IP address and the CIDR suffix with a slash.

4

Add explanation entry to the dictionary

Add a new entry to the cidr_notation dictionary with the key 'explanation' and the value 'CIDR notation shows the IP address followed by a slash and the number of bits for the network part.'

Computer Networks

ip_addresses = {
    '192.168.1.0': '255.255.255.0',
    '10.0.0.0': '255.0.0.0',
    '172.16.0.0': '255.240.0.0'
}
cidr_suffix = 24
cidr_notation = {}
for ip, mask in ip_addresses.items():
    cidr_notation[ip] = f"{ip}/{cidr_suffix}"
# Add the explanation entry below
# Your code here

Hint

Use the dictionary key assignment syntax to add the explanation string.

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