The answer is that the security group attached to the unreachable EC2 instances is not allowing inbound traffic from the on-premises network. This is because security groups are stateful firewalls that control traffic at the instance level; if the inbound rule does not explicitly permit traffic from the on-premises IP range, the packets are silently dropped, even though BGP routes are correctly advertised and the subnet route table points to the virtual private gateway. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your ability to distinguish between security groups and network ACLs, a common trap where engineers blame BGP or routing when the real issue is instance-level filtering. Remember that security groups default-deny all inbound traffic, so a missing allow rule for the on-premises CIDR will block traffic to specific instances while others remain reachable. Memory tip: “BGP gets the traffic there, but the security group decides if it gets in.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 company is expanding its on-premises data center to AWS using a hybrid cloud architecture. They have established an AWS Direct Connect connection with a private virtual interface to a VPC. The on-premises network uses BGP to exchange routes with the VPC. The network engineer notices that the on-premises network can reach some EC2 instances in the VPC but not others. All EC2 instances are in the same subnet (10.0.1.0/24) and have private IP addresses. The Direct Connect virtual interface is configured with the VPC CIDR (10.0.0.0/16) advertised to on-premises. The on-premises firewall logs show that traffic to the unreachable instances is being dropped. 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 security group attached to the unreachable instances is not allowing inbound traffic from the on-premises network.
Option D is correct because security groups are stateful and control inbound traffic; if the security group does not allow inbound traffic from on-premises, it will be dropped. Option A is wrong because the subnet route table has a route to the virtual private gateway, which is correct. Option B is wrong because the VPC CIDR is advertised. Option C is wrong because network ACLs are stateless and need both inbound and outbound rules; but if they were misconfigured, all instances would be affected.
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 route table does not have a route to the on-premises network.
Why it's wrong here
Wrong: The route to the virtual private gateway covers all on-premises traffic.
✗
The BGP session is not advertising the on-premises routes to the VPC.
Why it's wrong here
Wrong: The issue is on-premises cannot reach instances, not the other way.
✗
The network ACL on the subnet is blocking inbound traffic from on-premises.
Why it's wrong here
Wrong: If network ACL blocked, all instances would be unreachable.
✓
The security group attached to the unreachable instances is not allowing inbound traffic from the on-premises network.
Why this is correct
Correct: Security groups can be different per instance, causing inconsistent reachability.
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.
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group attached to the unreachable instances is not allowing inbound traffic from the on-premises network. — Option D is correct because security groups are stateful and control inbound traffic; if the security group does not allow inbound traffic from on-premises, it will be dropped. Option A is wrong because the subnet route table has a route to the virtual private gateway, which is correct. Option B is wrong because the VPC CIDR is advertised. Option C is wrong because network ACLs are stateless and need both inbound and outbound rules; but if they were misconfigured, all instances would be affected.
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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