- A
The subnet's route table has a route to an internet gateway.
Why wrong: The output does not show route tables; but if the instance is in a private subnet, the route table should not have a route to an IGW.
- B
The instance has an Elastic IP address attached.
Why wrong: Elastic IP would appear in the output; it does not, and a private subnet instance typically does not have an EIP.
- C
The instance has a public IP address assigned.
Why wrong: The output does not show a public IP; also, the instance is in a private subnet, so it should not have a public IP.
- D
The security group attached to the instance allows inbound traffic from 0.0.0.0/0.
The security group 'sg-web-app' might have an inbound rule allowing all traffic, making the instance accessible even in a private subnet if there is a NAT gateway or if the instance has a public IP.
Quick Answer
The answer is that the security group attached to the instance allows inbound traffic from 0.0.0.0/0. This is the most likely reason an EC2 instance in a private subnet becomes accessible from the internet, because security groups act as a virtual firewall at the instance level, and a rule permitting all inbound traffic overrides the subnet’s private designation. A private subnet lacks a direct route to an internet gateway, but if the security group opens port access to the entire internet, any traffic reaching the instance—whether through a NAT device, a load balancer, or an accidental public IP—can flow in. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding that security group rules and subnet routing are independent controls; a common trap is assuming a private subnet automatically blocks all internet traffic. Remember the memory tip: “A security group is a door, not a fence—if the door is wide open to 0.0.0.0/0, the subnet’s privacy doesn’t matter.”
SAP-C02 Continuous Improvement for Existing Solutions Practice Question
This SAP-C02 practice question tests your understanding of continuous improvement for existing solutions. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 Solutions Architect runs the above command for an EC2 instance that is part of an Auto Scaling group. The instance is in a private subnet and should only receive traffic from an Application Load Balancer. However, the instance is accessible from the internet. Based on the output, what is the most likely reason?
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 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
The security group attached to the instance allows inbound traffic from 0.0.0.0/0.
Option B is correct. The output shows the instance has a security group 'sg-web-app' attached to its network interface. If that security group allows inbound traffic from 0.0.0.0/0, the instance would be accessible from the internet. The subnet being private does not matter if a security group allows public access. Option A is wrong because the instance is in a private subnet, so it should not have a public IP. Option C is wrong because the output does not show a route table. Option D is wrong because the instance does not have a public IP attached; it's in a private subnet.
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 route table has a route to an internet gateway.
Why it's wrong here
The output does not show route tables; but if the instance is in a private subnet, the route table should not have a route to an IGW.
- ✗
The instance has an Elastic IP address attached.
Why it's wrong here
Elastic IP would appear in the output; it does not, and a private subnet instance typically does not have an EIP.
- ✗
The instance has a public IP address assigned.
Why it's wrong here
The output does not show a public IP; also, the instance is in a private subnet, so it should not have a public IP.
- ✓
The security group attached to the instance allows inbound traffic from 0.0.0.0/0.
Why this is correct
The security group 'sg-web-app' might have an inbound rule allowing all traffic, making the instance accessible even in a private subnet if there is a NAT gateway or if the instance has a public IP.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
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.
Trap categories for this question
Command / output trap
The output does not show route tables; but if the instance is in a private subnet, the route table should not have a route to an IGW.
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 SAP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this SAP-C02 question test?
Continuous Improvement for Existing Solutions — This question tests Continuous Improvement for Existing Solutions — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group attached to the instance allows inbound traffic from 0.0.0.0/0. — Option B is correct. The output shows the instance has a security group 'sg-web-app' attached to its network interface. If that security group allows inbound traffic from 0.0.0.0/0, the instance would be accessible from the internet. The subnet being private does not matter if a security group allows public access. Option A is wrong because the instance is in a private subnet, so it should not have a public IP. Option C is wrong because the output does not show a route table. Option D is wrong because the instance does not have a public IP attached; it's in a private subnet.
What should I do if I get this SAP-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 SAP-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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Last reviewed: Jun 20, 2026
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