CIDR blocks and IP addressing in AWS - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding CIDR Block Sizes

Which CIDR block provides the largest number of usable IP addresses?

A 10.0.0.0/24

B 10.0.0.0/22

C 10.0.0.0/26

D 10.0.0.0/30

💡 Hint

Smaller CIDR suffix means more IP addresses.

A /22 block has 1024 IP addresses, which is more than /24 (256), /26 (64), and /30 (4).

❓ Architecture

intermediate

Subnetting a VPC

You have a VPC with CIDR block 192.168.0.0/16. You want to create subnets that each have 256 IP addresses. Which subnet CIDR block should you use?

A 192.168.1.0/24

B 192.168.1.0/25

C 192.168.1.0/23

D 192.168.1.0/26

💡 Hint

Recall that a /24 subnet has 256 IP addresses.

A /24 subnet provides 256 IP addresses, matching the requirement exactly.

❓ security

advanced

Avoiding IP Overlaps in Peered VPCs

You have two VPCs peered together. VPC A uses 10.0.0.0/16 and VPC B uses 10.0.128.0/17. What issue will arise from this configuration?

A The CIDR blocks overlap, causing routing conflicts.

B Peering is not allowed between these CIDR blocks.

C No issue; the CIDR blocks do not overlap.

D VPC B's CIDR block is invalid for peering.

💡 Hint

Check if the IP ranges in the two CIDR blocks share any addresses.

10.0.128.0/17 covers 10.0.128.0 to 10.0.255.255, which is inside 10.0.0.0/16 range, causing overlap.

❓ service_behavior

advanced

AWS Reserved IP Addresses in a Subnet

In an AWS subnet with CIDR block 10.0.1.0/24, how many IP addresses are available for your instances?

A 253 IP addresses

B 256 IP addresses

C 251 IP addresses

D 250 IP addresses

💡 Hint

AWS reserves 5 IP addresses per subnet.

A /24 subnet has 256 IPs, but AWS reserves 5, leaving 251 usable IPs.

✅ Best Practice

expert

Designing a Multi-AZ VPC with Non-Overlapping Subnets

You need to design a VPC with CIDR block 172.31.0.0/16 across three Availability Zones (AZs). Each AZ requires two subnets: one public and one private, each with 4096 IP addresses. Which set of subnet CIDR blocks correctly meets these requirements without overlap?

A AZ1: 172.31.0.0/21 (public), 172.31.8.0/21 (private); AZ2: 172.31.16.0/21 (public), 172.31.24.0/21 (private); AZ3: 172.31.32.0/21 (public), 172.31.40.0/21 (private)

B AZ1: 172.31.0.0/19 (public), 172.31.8.0/20 (private); AZ2: 172.31.16.0/20 (public), 172.31.24.0/20 (private); AZ3: 172.31.32.0/20 (public), 172.31.40.0/20 (private)

C AZ1: 172.31.0.0/20 (public), 172.31.12.0/20 (private); AZ2: 172.31.24.0/20 (public), 172.31.36.0/20 (private); AZ3: 172.31.48.0/20 (public), 172.31.60.0/20 (private)

D AZ1: 172.31.0.0/20 (public), 172.31.16.0/20 (private); AZ2: 172.31.32.0/20 (public), 172.31.48.0/20 (private); AZ3: 172.31.64.0/20 (public), 172.31.80.0/20 (private)

💡 Hint

A /20 subnet has 4096 IP addresses. Ensure subnets do not overlap and fit within /16.

Option D assigns non-overlapping /20 subnets spaced by 16.0 increments, each with 4096 IPs, fitting within 172.31.0.0/16.

Practice

1. What does a CIDR block like 192.168.1.0/24 represent in AWS networking?

easy

A. A single IP address 192.168.1.24

B. An invalid IP address range

C. A subnet mask of 255.255.0.0

D. A range of IP addresses from 192.168.1.0 to 192.168.1.255

Solution

1. Step 1: Understand CIDR notation

   The number after the slash (/24) shows how many bits are fixed for the network part. Here, 24 bits fixed means the first 3 parts (192.168.1) are fixed.

2. Step 2: Calculate the IP range

   With 24 bits fixed, the last 8 bits can vary from 0 to 255, so the range is 192.168.1.0 to 192.168.1.255.

3. Final Answer:

   A range of IP addresses from 192.168.1.0 to 192.168.1.255 -> Option D

4. Quick Check:

   CIDR /24 means 256 addresses [OK]

Hint: Count bits after slash to find IP range size [OK]

Common Mistakes:

• Confusing CIDR with a single IP
• Misreading the subnet mask bits
• Assuming /24 means only 24 addresses

2. Which of the following is the correct CIDR notation for a subnet with 512 IP addresses?

easy

A. /23

B. /25

C. /22

D. /24

Solution

1. Step 1: Calculate bits needed for 512 addresses

   512 addresses require 9 bits (2^9 = 512) for host part.

2. Step 2: Determine CIDR prefix

   IPv4 has 32 bits total, so prefix = 32 - 9 = 23. So CIDR is /23.

3. Final Answer:

   /23 -> Option A

4. Quick Check:

   512 IPs = 2^(32-23) = 512 [OK]

Hint: Use 32 minus log2(IP count) for CIDR [OK]

Common Mistakes:

• Choosing /24 which gives only 256 addresses
• Confusing /22 with 1024 addresses
• Miscounting bits for hosts

3. Given the CIDR block 10.0.0.0/26, how many usable IP addresses are available for hosts?

medium

A. 64

B. 62

C. 32

D. 30

Solution

1. Step 1: Calculate total IPs in /26 block

   /26 means 32 - 26 = 6 bits for hosts, so total IPs = 2^6 = 64.

2. Step 2: Subtract network and broadcast addresses

   Two addresses are reserved (network and broadcast), so usable IPs = 64 - 2 = 62.

3. Final Answer:

   62 -> Option B

4. Quick Check:

   Usable IPs = total - 2 [OK]

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

Common Mistakes:

• Counting all IPs as usable
• Forgetting to subtract network and broadcast
• Mixing up prefix length and host bits

4. You have a VPC with CIDR block 172.16.0.0/16. You want to create two subnets without overlapping IPs. Which pair of CIDR blocks is valid?

medium

A. 172.16.0.0/17 and 172.16.128.0/17

B. 172.16.0.0/18 and 172.16.64.0/17

C. 172.16.0.0/16 and 172.16.0.0/17

D. 172.16.0.0/15 and 172.16.128.0/17

Solution

1. Step 1: Understand the VPC range

   172.16.0.0/16 covers IPs from 172.16.0.0 to 172.16.255.255.

2. Step 2: Check subnet ranges for overlap

   /17 splits the /16 into two halves: 172.16.0.0 to 172.16.127.255 and 172.16.128.0 to 172.16.255.255. These do not overlap.

3. Final Answer:

   172.16.0.0/17 and 172.16.128.0/17 -> Option A

4. Quick Check:

   Non-overlapping halves split /16 into two /17s [OK]

Hint: Split CIDR by increasing prefix to avoid overlap [OK]

Common Mistakes:

• Choosing overlapping CIDRs
• Using larger CIDR than VPC block
• Ignoring subnet mask sizes

5. You need to design a VPC with exactly 3 subnets: one public with 100 IPs, one private with 50 IPs, and one isolated with 25 IPs. Which CIDR block allocation fits best inside 10.0.0.0/24 without overlap?

hard

A. 10.0.0.0/24, 10.0.1.0/25, 10.0.2.0/26

B. 10.0.0.0/26, 10.0.0.64/26, 10.0.0.128/26

C. 10.0.0.0/25, 10.0.0.128/26, 10.0.0.192/27

D. 10.0.0.0/26, 10.0.0.64/27, 10.0.0.96/28

Solution

1. Step 1: Calculate needed CIDR for each subnet

   100 IPs need at least /25 (128 IPs), 50 IPs need /26 (64 IPs), 25 IPs need /27 (32 IPs).

2. Step 2: Assign CIDRs inside 10.0.0.0/24 without overlap

   10.0.0.0/25 covers 0-127, 10.0.0.128/26 covers 128-191, 10.0.0.192/27 covers 192-223. These fit perfectly without overlap.

3. Final Answer:

   10.0.0.0/25, 10.0.0.128/26, 10.0.0.192/27 -> Option C

4. Quick Check:

   Subnet sizes fit and sum within /24 [OK]

Hint: Match subnet size to nearest CIDR block, assign sequentially [OK]

Common Mistakes:

• Using CIDRs too small for IP needs
• Overlapping subnet ranges
• Assigning subnets outside VPC CIDR