The answer is the missing route to an internet gateway in the public subnet’s route table. Even with a correct security group allowing inbound traffic from the allowed IP CIDR on port 443, the Application Load Balancer cannot receive internet traffic if its subnet lacks a default route (0.0.0.0/0) pointing to an internet gateway. This is because the ALB’s elastic network interface relies on that route for return traffic to reach external clients; without it, packets from the internet are dropped at the subnet boundary. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that security groups are stateful and filter at the instance level, but routing is a separate, prerequisite layer—a common trap is to focus solely on security group rules while ignoring the subnet’s route table. A quick memory tip: “No IGW route, no internet shout” — always verify the subnet’s route table before blaming security groups for an ALB not accessible from the internet.
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. 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 uses AWS CloudFormation to deploy a multi-tier application. The template includes a VPC, public and private subnets, security groups, and an Application Load Balancer. The network team wants to ensure that the ALB can only accept traffic from a specific set of IP addresses. They add a security group rule that allows inbound traffic on port 443 from the allowed IP CIDR. However, after deployment, the ALB is not responding to requests from the allowed IPs. The team checks the security group and confirms the rule exists. They also verify that the ALB is in the public subnet and has a public DNS name. 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 public subnet does not have a route to an internet gateway.
The ALB must be in a public subnet with an internet gateway route for 0.0.0.0/0 to be accessible from the internet. If the subnet route table does not have that route, traffic from the internet cannot reach the ALB. Option A is the most likely. Option B is false because security groups are stateful. Option C is irrelevant as ALB uses listeners. Option D is not needed for internet-facing ALB.
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 security group rule is blocking return traffic; security groups are stateful.
Why it's wrong here
Security groups are stateful; return traffic is automatically allowed.
✗
The ALB listener is not configured to forward traffic to the target group.
Why it's wrong here
Would cause health check failures but not block initial connection.
✓
The public subnet does not have a route to an internet gateway.
Why this is correct
Without internet gateway, ALB cannot receive traffic from internet.
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 media company stores terabytes of video archives that are accessed once a year for audit purposes. Moving these objects to a cold storage tier (Azure Archive, S3 Glacier, or Google Nearline) costs a fraction of hot storage. Questions like this test whether you understand storage tiers, access frequency tradeoffs, and retrieval latency requirements.
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 Management and Operations — This question tests Network Management and Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The public subnet does not have a route to an internet gateway. — The ALB must be in a public subnet with an internet gateway route for 0.0.0.0/0 to be accessible from the internet. If the subnet route table does not have that route, traffic from the internet cannot reach the ALB. Option A is the most likely. Option B is false because security groups are stateful. Option C is irrelevant as ALB uses listeners. Option D is not needed for internet-facing ALB.
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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