ANS-C01 Network Implementation Practice Question

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?

Answer choices

Why each option matters

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 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.

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.
Why this is correct
NAT gateway security groups can block traffic.

Visual reference

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Last reviewed: Jun 20, 2026

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