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 with public and private subnets in two Availability Zones. They have a NAT gateway in each AZ for outbound internet access. They recently added a third AZ and created a new private subnet. Instances in the new private subnet cannot reach the internet. The route table for the new subnet has a default route (0.0.0.0/0) pointing to a NAT gateway in the same AZ. 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 NAT gateway in the new AZ does not have an Elastic IP address attached
Option A is correct because the NATgateway must have an Elastic IP (EIP) to provide internet connectivity. Without an EIP, the NAT gateway cannot translate private IPs for internet access. Option B is wrong because NAT gateways are not assigned to subnets; they are placed in public subnets. Option C is wrong because the route table is correctly configured. Option D is wrong because security group rules are for inbound/outbound traffic, but the NAT gateway itself does not have security groups.
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 attached to the NAT gateway blocks outbound traffic
Why it's wrong here
NAT gateways do not have security groups; they are controlled by route tables and network ACLs.
✗
The route table in the new private subnet does not have a route to the NAT gateway
Why it's wrong here
The route table has a default route pointing to the NAT gateway, so this is not the issue.
✓
The NAT gateway in the new AZ does not have an Elastic IP address attached
Why this is correct
A NATgateway requires an Elastic IP to function; without it, outbound traffic fails.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The NAT gateway is not in a public subnet in the new AZ
Why it's wrong here
NAT gateways must be in a public subnet, but the question does not indicate that it is not.
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 NAT gateway in the new AZ does not have an Elastic IP address attached — Option A is correct because the NAT gateway must have an Elastic IP (EIP) to provide internet connectivity. Without an EIP, the NAT gateway cannot translate private IPs for internet access. Option B is wrong because NAT gateways are not assigned to subnets; they are placed in public subnets. Option C is wrong because the route table is correctly configured. Option D is wrong because security group rules are for inbound/outbound traffic, but the NAT gateway itself does not have security groups.
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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