Quick Answer
The correct answer is the ALB in public subnets and the EC2 instances in private subnets. This configuration works because an Application Load Balancer must be placed in public subnets with internet-facing scheme to receive and route traffic from the internet, while the backend EC2 instances should reside in private subnets to remain inaccessible from the internet, enhancing security by forcing all inbound traffic through the ALB. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of load balancer subnet placement and the separation of public and private tiers within a VPC, a fundamental concept for designing secure, scalable architectures. A common trap is reversing the placement—placing the ALB in private subnets, which would block internet traffic, or putting EC2 instances in public subnets, which exposes them directly. Remember the memory tip: “ALB faces the web, EC2 hides in the crib”—the load balancer must be public-facing, while the instances stay safely private.
This ANS-C01 practice question tests your understanding of network implementation. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A company is deploying a web application in a VPC with an Application Load Balancer (ALB) in front of EC2 instances. The ALB must only accept traffic from the internet and forward it to the instances. Which subnet configuration is correct for the ALB and EC2 instances?
ALB in private subnets, EC2 in private subnets with a NAT gateway.
Why wrong: ALB would not receive internet traffic.
ALB in public subnets, EC2 in private subnets.
Standard architecture for internet-facing ALB.
ALB in public subnets, EC2 in public subnets.
Why wrong: Backend instances should be private for security.
ALB in private subnets, EC2 in public subnets.
Why wrong: ALB needs internet access.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
ALB in public subnets, EC2 in private subnets.
Option B is correct because ALBs must be in public subnets to receive internet traffic, and backend instances should be in private subnets for security. A puts both in public. C is reversed. D puts ALB in private.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
ALB in private subnets, EC2 in private subnets with a NAT gateway.
Why it's wrong here
ALB would not receive internet traffic.
ALB in public subnets, EC2 in private subnets.
Why this is correct
Standard architecture for internet-facing ALB.
Related concept
CIDR notation defines the prefix length.
ALB in public subnets, EC2 in public subnets.
Why it's wrong here
Backend instances should be private for security.
ALB in private subnets, EC2 in public subnets.
Why it's wrong here
ALB needs internet access.
Common exam traps
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
What to study next
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Network Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
The correct answer is: ALB in public subnets, EC2 in private subnets. — Option B is correct because ALBs must be in public subnets to receive internet traffic, and backend instances should be in private subnets for security. A puts both in public. C is reversed. D puts ALB in private.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
CIDR notation defines the prefix length.
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Last reviewed: Jun 20, 2026
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