The answer is that the VPC endpoint security group does not allow inbound HTTPS from the consumer's EC2 instances. This is correct because, in AWS PrivateLink, the security group attached to the VPC endpoint controls inbound traffic from the consumer’s resources to the endpoint service; if it lacks a rule permitting HTTPS from the instance CIDR, the connection will fail even though DNS resolution succeeds. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how security groups govern traffic flow at the endpoint interface, a common trap where candidates mistakenly blame routing or NAT gateways—but PrivateLink uses private IPs and local routing, so those are irrelevant. A key memory tip: think of the endpoint security group as the “front door” for inbound traffic from your instances; if the door is locked (no inbound rule), no amount of correct DNS or routing will open it.
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 uses AWS PrivateLink to access a SaaS application hosted in another AWS account. The SaaS provider has created a VPC endpoint service in their account. The consumer has created a VPC endpoint in their VPC. The consumer's VPC has a route table with a local route and a route to a NAT gateway. The VPC endpoint is associated with a security group that allows inbound HTTPS from the consumer's VPC CIDR. The consumer's EC2 instances can resolve the DNS name of the endpoint but cannot connect to the SaaS service. 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 VPC endpoint security group does not allow inbound HTTPS from the consumer's EC2 instances
For PrivateLink, the consumer's VPC endpoint should be associated with a security group that allows outbound traffic to the endpoint service. The security group for the endpoint controls inbound traffic to the endpoint from the consumer's instances. But the issue is that instances can resolve DNS but cannot connect. The most likely cause is that the endpoint's security group does not allow inbound HTTPS from the instances. Option B (NATgateway) is irrelevant because PrivateLink uses private IPs. Option C (route table) is wrong because the endpoint uses local routing via the endpoint's network interfaces. Option D (availability) is wrong because DNS resolution works.
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 consumer's route table routes traffic to the endpoint via the NAT gateway
Why it's wrong here
Traffic to endpoint uses local route via endpoint network interfaces.
✗
The VPC endpoint service is not available in the consumer's Availability Zone
The consumer's route table does not have a route to the VPC endpoint
Why it's wrong here
No specific route needed; endpoint uses local route.
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.
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 VPC endpoint security group does not allow inbound HTTPS from the consumer's EC2 instances — For PrivateLink, the consumer's VPC endpoint should be associated with a security group that allows outbound traffic to the endpoint service. The security group for the endpoint controls inbound traffic to the endpoint from the consumer's instances. But the issue is that instances can resolve DNS but cannot connect. The most likely cause is that the endpoint's security group does not allow inbound HTTPS from the instances. Option B (NAT gateway) is irrelevant because PrivateLink uses private IPs. Option C (route table) is wrong because the endpoint uses local routing via the endpoint's network interfaces. Option D (availability) is wrong because DNS resolution works.
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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