The answer is a security group or network ACL blocking traffic, because the route table already contains a valid entry for the peered VPC via pcx-, confirming routing is correctly configured. When VPC peering connectivity troubleshooting reveals that instances in a subnet cannot reach the peered VPC despite a proper route, the most likely cause is a restrictive security group or NACL on the source subnet, as these act as stateful and stateless firewalls respectively, filtering traffic before it ever leaves the subnet. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your ability to distinguish between routing misconfigurations and firewall rules—a common trap is to assume the peering connection itself is down or the route is missing, when in fact the route is present and the connection may be active. Remember the memory tip: “Route says go, but firewall says no.”
ANS-C01 Network Implementation Practice Question
This ANS-C01 practice question tests your understanding of network implementation. 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 network engineer examines the route table above. The VPC has a CIDR of 10.0.0.0/16. There is a VPC peering connection (pcx-...) to a VPC with CIDR 192.168.0.0/16. However, instances in this route table's subnet cannot communicate with the peered VPC. 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 or network ACL in the source subnet is blocking traffic.
Option D is correct because the route table shows a route to the peered VPC via pcx, so routing seems configured. The issue is likely that the security groups or NACLs in the source subnet are blocking traffic. Option A is wrong because the route exists. Option B is wrong because the peering connection may be active; the issue is not shown. Option C is wrong because the route table is associated with the subnet (implied by the question).
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 route table is not associated with the subnet.
Why it's wrong here
The subnet uses this route table, as implied.
✗
The VPC peering connection is in 'pending-acceptance' state.
Why it's wrong here
The command does not show the state, but likely it is active if the route exists.
✓
The security group or network ACL in the source subnet is blocking traffic.
Why this is correct
Even with correct routing, security groups/NACLs can block traffic.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The route to the peered VPC is missing from the route table.
Why it's wrong here
The route is present.
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 command does not show the state, but likely it is active if the route exists.
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 Implementation — This question tests Network Implementation — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group or network ACL in the source subnet is blocking traffic. — Option D is correct because the route table shows a route to the peered VPC via pcx, so routing seems configured. The issue is likely that the security groups or NACLs in the source subnet are blocking traffic. Option A is wrong because the route exists. Option B is wrong because the peering connection may be active; the issue is not shown. Option C is wrong because the route table is associated with the subnet (implied by the question).
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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