The answer is that the NAT gateway failed because the Elastic IP (EIP) is already associated with another instance. A NAT gateway requires a unique, unassociated Elastic IP to function; if the EIP is already attached to an EC2 instance or another resource, the allocation fails, immediately placing the NAT gateway into a 'failed' state. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of NAT gateway dependencies and common misconfigurations—a frequent trap is assuming a routing issue or pending state, but the key clue is the presence of an allocation ID alongside a failed status. Remember that a NAT gateway in a public subnet with a route to an internet gateway will only fail due to resource conflicts, not network path problems. Memory tip: "One EIP, one gateway—no sharing allowed."
ANS-C01 Network Management and Operations Practice Question
This ANS-C01 practice question tests your understanding of network management and operations. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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
Refer to the exhibit. A network engineer created a NAT gateway in a public subnet, but its state shows 'failed'. 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 Elastic IP is already associated with another instance
The correct answer is B because the NATgateway address has a public IP but the allocation ID is present; a common reason for failure is that the Elastic IP is already associated with another resource. Option A is wrong because the subnet is public (has route to IGW). Option C is wrong because the state is 'failed', not 'pending'. Option D is wrong because the NAT gateway is in a public subnet; it should have a route to IGW.
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 subnet does not have a route to an internet gateway
Why it's wrong here
The presence of a public IP suggests the subnet is public and has a route to IGW.
✗
The subnet is private
Why it's wrong here
The subnet is public (has public IP allocated).
✗
The NAT gateway is still being created
Why it's wrong here
State is 'failed', not 'pending'.
✓
The Elastic IP is already associated with another instance
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 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 Elastic IP is already associated with another instance — The correct answer is B because the NAT gateway address has a public IP but the allocation ID is present; a common reason for failure is that the Elastic IP is already associated with another resource. Option A is wrong because the subnet is public (has route to IGW). Option C is wrong because the state is 'failed', not 'pending'. Option D is wrong because the NAT gateway is in a public subnet; it should have a route to IGW.
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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