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.
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
The public subnet does not have a route to the internet gateway.
Why wrong: Public subnets typically have IGW route, but NAT is in public subnet and should have IGW route.
B
The private subnet route table does not have a route to the NAT gateway.
Why wrong: The route is present.
C
The NAT gateway is not in the same subnet as the private instances.
Why wrong: NAT can be in any public subnet, not necessarily same as private.
D
The NAT gateway's security group is blocking traffic from the private subnet.
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.
The NATgateway has a security group that by default denies all inbound traffic. For instances in the private subnet to send traffic to the internet, the NAT gateway's security group must allow inbound traffic from the private subnet's CIDR (or from the private subnet's network ACLs) and allow outbound traffic to the internet. Option D is correct because this is the most likely issue: the security group is blocking traffic. Option A is incorrect because the public subnet route to the IGW is presumptively configured correctly for the NAT gateway to reach the internet. Option B is incorrect because the route to the NAT gateway is present in the private route table. Option C is incorrect because the NAT gateway does not need to be in the same subnet as the private instances; it must be in a public subnet.
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.
Visual reference
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. — The NAT gateway has a security group that by default denies all inbound traffic. For instances in the private subnet to send traffic to the internet, the NAT gateway's security group must allow inbound traffic from the private subnet's CIDR (or from the private subnet's network ACLs) and allow outbound traffic to the internet. Option D is correct because this is the most likely issue: the security group is blocking traffic. Option A is incorrect because the public subnet route to the IGW is presumptively configured correctly for the NAT gateway to reach the internet. Option B is incorrect because the route to the NAT gateway is present in the private route table. Option C is incorrect because the NAT gateway does not need to be in the same subnet as the private instances; it must be in a public subnet.
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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