The answer is that the subnet’s route table does not have a route to the internet gateway. Even though the EC2 instance has a public IP address and the security group allows all outbound traffic, the subnet itself must have a default route (0.0.0.0/0) pointing to an internet gateway for traffic to leave the VPC. Without that route, packets from the instance have no path to the internet, regardless of the instance’s public IP or security group rules. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding that internet connectivity depends on three layers: the instance’s public IP, the security group’s outbound rules, and—most critically—the subnet’s route table. A common trap is assuming a public IP alone guarantees internet access, but the route table is the gatekeeper. Memory tip: “IP gives the address, but the route gives the road.”
SAP-C02 Design for New Solutions Practice Question
This SAP-C02 practice question tests your understanding of design for new solutions. 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.
Network Topology
A solutions architect is troubleshooting an issue where an EC2 instance cannot connect to the internet. The output of the describe-instances CLI command is shown in the exhibit. The instance is in a VPC with a public subnet that has a route table with a default route pointing to an internet gateway. The security group allows outbound traffic to 0.0.0.0/0. What is the MOST likely cause of the problem?
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 subnet's route table does not have a route to the internet gateway.
Option B is correct. The instance has a public IP (54.123.45.67), so it should be able to reach the internet. However, if the subnet does not have a route to the internet gateway, the instance cannot connect. Option A is wrong because the instance has a public IP. Option C is wrong because the security group allows outbound traffic. Option D is wrong because the instance state is running.
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 does not have a route to the internet gateway.
Why this is correct
Even with a public IP, the subnet must have a route to the internet gateway.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
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 shows a public IP: 54.123.45.67.
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 SAP-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Design for New Solutions — This question tests Design for New Solutions — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The subnet's route table does not have a route to the internet gateway. — Option B is correct. The instance has a public IP (54.123.45.67), so it should be able to reach the internet. However, if the subnet does not have a route to the internet gateway, the instance cannot connect. Option A is wrong because the instance has a public IP. Option C is wrong because the security group allows outbound traffic. Option D is wrong because the instance state is running.
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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Question Discussion
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