The answer is that the instance is in a private subnet without a NAT gateway or route to an internet gateway. This is correct because while the security group egress rule is fully permissive, allowing all outbound traffic, a private subnet has no direct path to the internet; it requires a NAT gateway in a public subnet with a route table entry pointing to that NAT device for outbound connectivity. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of the distinction between security group rules (stateful, permissive) and subnet routing—a common trap is assuming a permissive security group alone grants internet access, when the real bottleneck is the missing NAT gateway route. Remember the memory tip: security groups control what traffic is allowed, but route tables control where traffic can go; for private subnet internet access, you need both a NAT gateway and a route pointing to it.
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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 CloudFormation stack was created with the above snippet. An administrator notices that the EC2 instance can receive HTTP traffic from the internet, but cannot access the internet itself (e.g., to download updates). What is the most likely cause?
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 instance is in a private subnet without a NAT gateway or route to an internet gateway.
Option A is correct because the security group egress rule allows all traffic (protocol -1) to 0.0.0.0/0, so outbound is allowed. The issue is likely that the instance is in a private subnet without a NATgateway or internet gateway route. Option B is wrong because the egress rule is permissive. Option C is wrong because the ingress rule allows HTTP. Option D is wrong because the security group is correctly associated.
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 security group egress rule restricts outbound traffic to only HTTP.
Why it's wrong here
The egress rule allows all traffic (-1), so it is not restrictive.
✓
The instance is in a private subnet without a NAT gateway or route to an internet gateway.
Why this is correct
Even with permissive security group rules, if the subnet has no route to the internet, outbound traffic fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security group is not attached to the instance properly.
Why it's wrong here
The snippet correctly attaches the security group via SecurityGroupIds.
✗
The security group ingress rule only allows HTTP from the internet, but not other protocols needed for updates.
Why it's wrong here
Ingress rules do not affect outbound traffic; outbound is controlled by egress rules.
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The instance is in a private subnet without a NAT gateway or route to an internet gateway. — Option A is correct because the security group egress rule allows all traffic (protocol -1) to 0.0.0.0/0, so outbound is allowed. The issue is likely that the instance is in a private subnet without a NAT gateway or internet gateway route. Option B is wrong because the egress rule is permissive. Option C is wrong because the ingress rule allows HTTP. Option D is wrong because the security group is correctly associated.
What should I do if I get this ANS-C01 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 ANS-C01 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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