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Question 1,138 of 1,546

Networking and Content Delivery →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is to add an inbound rule to the private subnet NACL allowing ephemeral ports (1024-65535) from the public subnet CIDR (10.0.1.0/24). This is correct because when an Application Load Balancer forwards traffic to targets in a private subnet, it uses random ephemeral ports for the return traffic, and Network ACLs are stateless—they require explicit inbound rules for these ports, unlike security groups which automatically track connection state. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of the critical difference between stateful security groups and stateless NACLs, a common trap where candidates assume a security group rule is sufficient. The key insight is that while the ALB’s security group allows traffic, the private subnet’s NACL must also permit the ephemeral port range from the ALB’s subnet to avoid blocking health checks and client requests. Remember the memory tip: “NACLs need the return path—allow ephemeral from the ALB’s CIDR.”

SOA-C02 Networking and Content Delivery Practice Question

This SOA-C02 practice question tests your understanding of networking and content delivery. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 has a production application running on EC2 instances behind an Application Load Balancer (ALB) in a VPC. The application uses an RDS MySQL database in the same VPC. The SysOps team recently implemented a change to the network ACLs to improve security. After the change, the application became unreachable from the internet, but the EC2 instances can still communicate with the RDS database. The ALB is in a public subnet, and the EC2 instances and RDS are in private subnets. The ALB's security group allows inbound HTTP/HTTPS from 0.0.0.0/0. The EC2 instances' security group allows inbound from the ALB's security group. The RDS security group allows inbound from the EC2 instances' security group. The network ACLs for the public subnet allow inbound HTTP/HTTPS from 0.0.0.0/0 and all outbound traffic. The network ACLs for the private subnets were modified to deny all inbound traffic except from the public subnet CIDR (10.0.1.0/24) and allow all outbound traffic. Which change should be made to restore internet access to the application?

Question 1hardmultiple choice

Study the full ACL explanation →

A
Add a route to the private subnet route table pointing 0.0.0.0/0 to the Internet Gateway.
Why wrong: Private subnets should not have direct internet access; this would bypass the ALB.
B
Modify the EC2 instance security group to allow inbound traffic from 0.0.0.0/0 on port 80.
Why wrong: The EC2 instances should only receive traffic from the ALB, not directly from internet.
C
Add an inbound rule to the private subnet NACL to allow ephemeral ports (1024-65535) from the public subnet CIDR (10.0.1.0/24).
The ALB uses ephemeral ports to forward traffic to targets; the NACL must allow return traffic.
D
Add an inbound rule to the public subnet NACL to allow HTTP/HTTPS from the ALB's security group.
Why wrong: The public subnet NACL already allows HTTP/HTTPS; the issue is in the private subnet.

Full breakdown with real-world context →

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

✓

Add an inbound rule to the private subnet NACL to allow ephemeral ports (1024-65535) from the public subnet CIDR (10.0.1.0/24).

Option C is correct. The private subnet NACL should allow inbound traffic from the ALB's security group, but NACLs do not support security group references; they use CIDR. The ALB's subnet CIDR is 10.0.1.0/24, but the ALB also uses ephemeral ports for health checks and traffic. However, the issue is that the private subnet NACL currently denies all inbound except from 10.0.1.0/24, which should allow traffic from the ALB. But the problem states that EC2 can communicate with RDS, so the NACL is not blocking that. The application is unreachable from the internet, meaning the ALB cannot forward traffic to EC2. The ALB's health checks might be failing because the private subnet NACL is blocking health check traffic from the ALB on ephemeral ports. The correct action is to allow inbound traffic on ephemeral ports from the ALB subnet. Option A is wrong because the public subnet NACL already allows HTTP/HTTPS inbound. Option B is wrong because the EC2 security group is correct. Option D is wrong because adding a route to the private subnet would not help; the issue is NACL.

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

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✗
Add a route to the private subnet route table pointing 0.0.0.0/0 to the Internet Gateway.
Why it's wrong here
Private subnets should not have direct internet access; this would bypass the ALB.
✗
Modify the EC2 instance security group to allow inbound traffic from 0.0.0.0/0 on port 80.
Why it's wrong here
The EC2 instances should only receive traffic from the ALB, not directly from internet.
✓
Add an inbound rule to the private subnet NACL to allow ephemeral ports (1024-65535) from the public subnet CIDR (10.0.1.0/24).
Why this is correct
The ALB uses ephemeral ports to forward traffic to targets; the NACL must allow return traffic.
Related concept
CIDR notation defines the prefix length.
✗
Add an inbound rule to the public subnet NACL to allow HTTP/HTTPS from the ALB's security group.
Why it's wrong here
The public subnet NACL already allows HTTP/HTTPS; the issue is in the private subnet.

Common exam traps

Common exam trap: usable hosts are not the same as total addresses

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

How to think about this question

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.

KKey Concepts to Remember

CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.

TExam Day Tips

Write the block size before choosing the subnet.
Check whether the question asks for hosts, subnets or a specific address range.
Do not confuse /24, /25, /26 and /27 host counts.

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

How this comes up in practice

A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.

What to study next

Got this wrong? Here's your next step.

Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this SOA-C02 question test?

Networking and Content Delivery — This question tests Networking and Content Delivery — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: Add an inbound rule to the private subnet NACL to allow ephemeral ports (1024-65535) from the public subnet CIDR (10.0.1.0/24). — Option C is correct. The private subnet NACL should allow inbound traffic from the ALB's security group, but NACLs do not support security group references; they use CIDR. The ALB's subnet CIDR is 10.0.1.0/24, but the ALB also uses ephemeral ports for health checks and traffic. However, the issue is that the private subnet NACL currently denies all inbound except from 10.0.1.0/24, which should allow traffic from the ALB. But the problem states that EC2 can communicate with RDS, so the NACL is not blocking that. The application is unreachable from the internet, meaning the ALB cannot forward traffic to EC2. The ALB's health checks might be failing because the private subnet NACL is blocking health check traffic from the ALB on ephemeral ports. The correct action is to allow inbound traffic on ephemeral ports from the ALB subnet. Option A is wrong because the public subnet NACL already allows HTTP/HTTPS inbound. Option B is wrong because the EC2 security group is correct. Option D is wrong because adding a route to the private subnet would not help; the issue is NACL.

What should I do if I get this SOA-C02 question wrong?

What is the key concept behind this question?

CIDR notation defines the prefix length.

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Last reviewed: Jun 20, 2026

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