The answer is the ALB security group not allowing inbound traffic from VPC A. This is because an internet-facing Application Load Balancer is designed to receive traffic via its public IP address from the internet, not from private IP addresses over a VPC peering connection. When instances in VPC A send traffic to the ALB in VPC B, the packets arrive with private source IPs from VPC A’s CIDR range, which the ALB’s security group blocks unless explicitly permitted. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how VPC peering troubleshooting ALB connectivity differs from basic ICMP reachability—ping works because it uses private IPs directly to instances, but ALB traffic fails due to security group rules that assume internet-sourced traffic. A common trap is assuming route tables are the issue when they are correctly configured, or forgetting that internet-facing ALBs do not automatically accept private IP traffic from peered VPCs. Memory tip: “Ping peers, but ALB needs a permit—private IPs don’t get a free pass.”
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.
A company has a VPC peering connection between VPC A (10.0.0.0/16) and VPC B (10.1.0.0/16). Both VPCs have route tables that include routes to each other's CIDR blocks via the peering connection. Instances in VPC A can ping instances in VPC B, but traffic to an Application Load Balancer (ALB) in VPC B fails. The ALB is in public subnets with internet-facing scheme. 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 ALB security group does not allow inbound traffic from VPC A
Option C is correct because an internet-facing ALB expects traffic to come from the internet via its public IP. Traffic from a peered VPC uses private IPs, which the ALB's security group may not allow unless explicitly permitted. Option A is wrong because route tables are configured correctly. Option B is wrong because the ALB can be reached from the internet. Option D is wrong because VPC peering does not affect the ALB's public accessibility.
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 VPC peering connection does not support transitive routing for ALBs
Why it's wrong here
VPC peering supports direct connectivity; ALB can be accessed via private IP.
✗
The ALB is internet-facing and cannot be accessed from a peered VPC
Why it's wrong here
Internet-facing ALBs can be accessed from peered VPCs via private IPs with proper security group rules.
✗
The route tables in VPC B do not have a route back to VPC A
Why it's wrong here
Route tables include routes to each other's CIDR blocks.
✓
The ALB security group does not allow inbound traffic from VPC A
Why this is correct
The security group must allow inbound traffic from the peered VPC's CIDR.
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 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 ALB security group does not allow inbound traffic from VPC A — Option C is correct because an internet-facing ALB expects traffic to come from the internet via its public IP. Traffic from a peered VPC uses private IPs, which the ALB's security group may not allow unless explicitly permitted. Option A is wrong because route tables are configured correctly. Option B is wrong because the ALB can be reached from the internet. Option D is wrong because VPC peering does not affect the ALB's public accessibility.
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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