The answer is the subnet’s network ACL blocking outbound ephemeral ports. This is correct because network ACLs are stateless, meaning they require explicit rules for both inbound and outbound traffic in each direction. Even if the security group allows outbound HTTPS, a custom NACL can block the high-numbered ephemeral ports (1024-65535) that the operating system uses to initiate the return traffic, effectively preventing the EC2 instance from completing the connection to the internet through the NAT Gateway. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of the stateless versus stateful distinction between NACLs and security groups—a common trap is assuming that allowing outbound traffic on the security group is sufficient, while forgetting that the NACL must also permit the response traffic. Remember the mnemonic: “NACL is stateless, so you must let traffic back in; SG is stateful, so it tracks the conversation.”
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 SysOps administrator is troubleshooting an issue where an EC2 instance in a private subnet cannot connect to the internet. The instance has a security group allowing outbound HTTPS traffic. The subnet’s route table has a default route (0.0.0.0/0) to a NAT Gateway. The NAT Gateway is in a public subnet with an Elastic IP and a route to an internet gateway. What is a likely cause of the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The subnet’s network ACL is blocking outbound ephemeral ports.
Option D is correct because the network ACL (NACL) is stateless and must allow both inbound and outbound traffic. Even if the default NACL allows all outbound, a custom NACL might block outbound ephemeral ports. Option A is wrong because security groups are stateful. Option B is wrong because the NATGateway has an EIP. Option C is wrong because the subnet is private, no IGW.
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.
✓
The subnet’s network ACL is blocking outbound ephemeral ports.
Why this is correct
NACL must allow outbound ephemeral ports (1024-65535) for responses.
The private subnet has a route to an internet gateway.
Why it's wrong here
Private subnets should not have IGW routes.
✗
The security group is blocking inbound traffic from the NAT Gateway.
Why it's wrong here
Security groups are stateful; outbound is allowed.
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 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
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.
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: The subnet’s network ACL is blocking outbound ephemeral ports. — Option D is correct because the network ACL (NACL) is stateless and must allow both inbound and outbound traffic. Even if the default NACL allows all outbound, a custom NACL might block outbound ephemeral ports. Option A is wrong because security groups are stateful. Option B is wrong because the NAT Gateway has an EIP. Option C is wrong because the subnet is private, no IGW.
What should I do if I get this SOA-C02 question wrong?
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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