CIDR notation in Computer Networks - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does CIDR stand for?

CIDR stands for Classless Inter-Domain Routing. It is a method for allocating IP addresses and routing Internet Protocol packets.

beginner

Explain the format of CIDR notation.

CIDR notation combines an IP address and a suffix indicating the number of bits used for the network part. It looks like this: 192.168.1.0/24, where /24 means the first 24 bits are the network portion.

intermediate

How does CIDR improve IP address allocation compared to older methods?

CIDR allows flexible division of IP address space without fixed classes, reducing wasted addresses and improving routing efficiency by aggregating routes.

beginner

What does the suffix number in CIDR notation represent?

The suffix number shows how many bits of the IP address are used to identify the network. For example, /24 means 24 bits are for the network, and the rest are for hosts.

intermediate

Given the CIDR notation 10.0.0.0/8, how many IP addresses are in this network?

A /8 means 8 bits for the network and 24 bits for hosts. So, there are 2^24 = 16,777,216 possible IP addresses in this network.

What does the number after the slash in CIDR notation indicate?

A Number of bits used for the network part

B Number of hosts in the network

C Number of subnets

D Number of IP addresses available

The number after the slash shows how many bits are used to identify the network portion of the IP address.

Which of these is a valid CIDR notation?

A 192.168.1.0/24

B 192.168.1.0-24

C 192.168.1.0#24

D 192.168.1.0:24

CIDR notation uses a slash '/' followed by the number of network bits.

CIDR helps to reduce which of the following?

A Internet speed

B IP address waste

C Number of devices

D Physical cable length

CIDR allows flexible IP address allocation, reducing wasted addresses.

What is the host portion in the IP address 172.16.0.0/16?

A All 32 bits

B First 16 bits

C Last 16 bits

D No host portion

With /16, the first 16 bits are network, so the last 16 bits are for hosts.

Which of these is NOT a benefit of CIDR?

A More efficient routing

B Route aggregation

C Better IP address utilization

D Fixed class-based addressing

CIDR replaces fixed class-based addressing with flexible allocation.

Describe what CIDR notation is and how it is used to represent IP networks.

Think about how IP addresses are grouped and shown in a shorter form.

Explain how CIDR improves the allocation and routing of IP addresses compared to older class-based methods.

Consider the problems with fixed classes and how CIDR solves them.

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