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.