The answer is that the NAT gateway is missing an Elastic IP address. A NAT gateway requires an associated Elastic IP to perform source network address translation (SNAT), which replaces the private IP of outbound traffic from the private subnet with a public IP so the internet can route replies back. Without this Elastic IP, the gateway cannot establish outbound connectivity, even if the route table and subnet configurations are correct. On the AWS Certified Security Specialty SCS-C02 exam, this scenario tests your understanding of NAT gateway prerequisites versus common misconfigurations like security group rules or route table entries. A frequent trap is assuming the NAT gateway itself needs a route to the internet gateway, but that route belongs to its public subnet, not the gateway. Remember the mnemonic: “No EIP, no internet path.”
SCS-C02 Infrastructure Security Practice Question
This SCS-C02 practice question tests your understanding of infrastructure security. 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 security engineer is troubleshooting why an EC2 instance in a private subnet cannot access the internet through a NAT gateway. The route table for the private subnet has a default route pointing to the NAT gateway. The NAT gateway is in a public subnet with a route to an internet gateway. What is the most likely cause of the issue?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The NAT gateway does not have an Elastic IP address associated.
Option C is correct because the NATgateway must have an associated Elastic IP to communicate with the internet. Option A is wrong because the security group of the NAT gateway must allow outbound traffic, but the default is permissive. Option B is wrong because the NAT gateway itself does not need a route to the internet gateway; its public subnet does. Option D is wrong because security groups are not applied to NAT gateways in the same way; they are applied to the ENI.
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 NAT gateway does not have an Elastic IP address associated.
Why this is correct
A NATgateway requires an Elastic IP for outbound traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The EC2 instance's security group is blocking outbound traffic.
Why it's wrong here
The security group outbound rules would affect the instance, but the issue is at the NATgateway level.
✗
The NAT gateway's security group is blocking outbound traffic.
Why it's wrong here
Security groups for NAT gateways have default outbound allow all; this is unlikely.
✗
The NAT gateway does not have a route to the internet gateway in its route table.
Why it's wrong here
The NATgateway uses the route table of the subnet it is in; if the subnet has a route to IGW, it works.
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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The NAT gateway does not have an Elastic IP address associated. — Option C is correct because the NAT gateway must have an associated Elastic IP to communicate with the internet. Option A is wrong because the security group of the NAT gateway must allow outbound traffic, but the default is permissive. Option B is wrong because the NAT gateway itself does not need a route to the internet gateway; its public subnet does. Option D is wrong because security groups are not applied to NAT gateways in the same way; they are applied to the ENI.
What should I do if I get this SCS-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 SCS-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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