ANS-C01 Network Design Practice Question
A company has a VPC with a CIDR of 10.0.0.0/16. It has two public subnets (10.0.1.0/24 and 10.0.2.0/24) and two private subnets (10.0.3.0/24 and 10.0.4.0/24). The company hosts a web application on EC2 instances in the private subnets behind an Application Load Balancer (ALB) in the public subnets. The ALB is internet-facing. The company wants to add a second ALB in the same VPC for a different application but using the same public subnets. The new ALB also needs to be internet-facing. However, when the company tries to create the new ALB, they receive an error: 'The subnet 'subnet-xxxxxxxx' does not have enough free IP addresses to satisfy the request.' The network engineer checks the subnets and finds that the public subnets have only 2 free IP addresses each. The private subnets have plenty of free IP addresses. The company wants to resolve this error without changing the architecture of the existing applications. What should the network engineer do?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Create new public subnets with larger CIDR blocks (e.g., /23) and associate them with the same route table.
The error indicates insufficient free IP addresses in the public subnets. Creating new public subnets with larger CIDR blocks (e.g., /23) provides more IP addresses without affecting the existing subnets or architecture. Option B is incorrect because private subnets cannot host an internet-facing ALB (they lack direct internet access and cannot assign public IPs). Option C would reduce available IPs and disrupt existing resources. Option D does not free up IP addresses in the subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Create new public subnets with larger CIDR blocks (e.g., /23) and associate them with the same route table.
Why this is correct
New public subnets provide more IP addresses.
- ✗
Use the private subnets for the new ALB and add a NAT gateway for internet access.
Why it's wrong here
Internet-facing ALBs must be in public subnets.
- ✗
Reduce the size of the existing public subnets by modifying their CIDR blocks.
Why it's wrong here
Cannot modify CIDR of existing subnets without recreating.
- ✗
Launch a NAT gateway in one public subnet to free up IP addresses.
Why it's wrong here
NAT gateway consumes an IP address, not frees.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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