ANS-C01 Network Design Practice Question
A company has a VPC with a CIDR block of 10.0.0.0/16. They have two subnets: a public subnet (10.0.1.0/24) and a private subnet (10.0.2.0/24). They launch an Amazon RDS for MySQL DB instance in the private subnet. The DB instance needs to be accessed by an EC2 instance in the public subnet. The security group for the DB instance allows inbound traffic on port 3306 from the security group of the EC2 instance. However, the EC2 instance cannot connect to the DB instance. What is the most likely cause?
⚠ Common exam trap
Candidates often assume that security group misconfigurations or missing public IPs are the cause, but they overlook the stateless nature of network ACLs. In a VPC, subnets automatically have a local route, so routing is not an issue. The real trap is failing to check the network ACL for the private subnet when inbound traffic is blocked.
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 network ACL for the private subnet is blocking inbound traffic
The most likely cause is that the network ACL (NACL) for the private subnet is blocking inbound traffic on port 3306. In Amazon VPC, each subnet has a default NACL that allows all inbound and outbound traffic by default, but if the NACL has been modified to block inbound traffic on port 3306, the connection from the EC2 instance will be denied. Security groups are stateful and allow return traffic automatically, but NACLs are stateless and require explicit rules for both inbound and outbound traffic. The private subnet's route table automatically includes a local route to the entire VPC CIDR (10.0.0.0/16), which cannot be deleted, so a missing route is not a possible cause. The DB instance does not need a public IP for VPC internal communication, and the EC2 security group likely allows all outbound traffic by default.
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 private subnet does not have a route to the public subnet
Why it's wrong here
Incorrect. The private subnet's route table automatically includes a local route for the entire VPC CIDR (10.0.0.0/16), which cannot be deleted. Therefore, the private subnet always has a route to the public subnet. A missing route is not a possible cause.
- ✓
The network ACL for the private subnet is blocking inbound traffic
Why this is correct
Correct. The network ACL for the private subnet is stateless and must allow inbound traffic on port 3306 from the public subnet. If it has been modified to block this traffic, the connection will fail. By default, NACLs allow all traffic, but custom rules may block it.
- ✗
The security group for the EC2 instance does not allow outbound traffic
Why it's wrong here
Incorrect. Security groups are stateful and allow all outbound traffic by default. Unless the outbound rule has been explicitly removed, the EC2 instance can initiate outbound traffic to the DB instance on port 3306.
- ✗
The DB instance does not have a public IP address
Why it's wrong here
Incorrect. For communication within a VPC, instances communicate using private IP addresses. The DB instance does not need a public IP address because the EC2 instance is in the same VPC.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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