The answer is the NAT gateway’s security group blocking traffic from the private subnet. This is correct because a NAT gateway in a public subnet functions as a bridge for outbound internet traffic, and its attached security group must explicitly allow inbound traffic from the private subnet’s CIDR range as well as outbound traffic to 0.0.0.0/0. Without these rules, the security group acts as a stateful firewall that drops the return traffic, even though the route table correctly points to the NAT gateway and the Elastic IP is attached. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding of how security groups interact with managed NAT gateways—a common trap is assuming NAT gateways ignore security groups, but they do not. Remember the mnemonic: NAT needs two rules—in from private, out to world.
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.
Exhibit
Refer to the exhibit. The following is a CloudFormation snippet:
PrivateSubnetRouteTable:
Type: AWS::EC2::RouteTable
Properties:
VpcId: !Ref VPC
PrivateRoute:
Type: AWS::EC2::Route
Properties:
RouteTableId: !Ref PrivateSubnetRouteTable
DestinationCidrBlock: 0.0.0.0/0
NatGatewayId: !Ref NatGateway
PrivateSubnet:
Type: AWS::EC2::Subnet
Properties:
VpcId: !Ref VPC
CidrBlock: 10.0.1.0/24
RouteTableId: !Ref PrivateSubnetRouteTable
A company is using CloudFormation to deploy a VPC. The private subnet route table has a route to a NAT gateway. However, instances in the private subnet cannot access the internet. The NAT gateway is in a public subnet and has an attached Elastic IP. What is the most likely issue?
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's security group is blocking traffic from the private subnet.
Option A is correct because the NATgateway's security group must allow inbound traffic from the private subnet and outbound to the internet. B is incorrect because the route is present. C is incorrect because public subnets don't need NAT. D is incorrect because the route points to NAT.
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 public subnet does not have a route to the internet gateway.
Why it's wrong here
Public subnets typically have IGW route, but NAT is in public subnet and should have IGW route.
✗
The private subnet route table does not have a route to the NAT gateway.
Why it's wrong here
The route is present.
✗
The NAT gateway is not in the same subnet as the private instances.
Why it's wrong here
NAT can be in any public subnet, not necessarily same as private.
✓
The NAT gateway's security group is blocking traffic from the private subnet.
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's security group is blocking traffic from the private subnet. — Option A is correct because the NAT gateway's security group must allow inbound traffic from the private subnet and outbound to the internet. B is incorrect because the route is present. C is incorrect because public subnets don't need NAT. D is incorrect because the route points to NAT.
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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