ANS-C01 Network Design Practice Question
A company has a VPC with CIDR 10.0.0.0/16. They have two subnets: subnet A (10.0.1.0/24) and subnet B (10.0.2.0/24). An EC2 instance in subnet A needs to communicate with an RDS database in subnet B. Both subnets have network ACLs that allow all inbound and outbound traffic. However, the instance cannot connect to the database. Which TWO configuration changes could solve this issue? (Choose TWO.)
⚠ Common exam trap
AWS often tests the misconception that network ACLs alone control all traffic flow, but candidates forget that security groups must also allow inbound traffic, and that route tables must have a local route for intra-VPC communication.
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
✓
Ensure that the route tables associated with both subnets have a local route for the VPC CIDR.
The local route for the VPC CIDR (10.0.0.0/16) is automatically added to the main route table, but if custom route tables are associated with subnets A and B, they must explicitly include this local route to enable routing between subnets within the same VPC. Without it, traffic from the EC2 instance in subnet A cannot reach the RDS database in subnet B, even if network ACLs allow all traffic.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Attach an Internet Gateway to the VPC.
Why it's wrong here
Not needed for internal communication.
- ✓
Ensure that the route tables associated with both subnets have a local route for the VPC CIDR.
Why this is correct
Local route is needed for intra-VPC communication.
- ✗
Create a VPC peering connection between the subnets.
Why it's wrong here
VPC peering is for different VPCs, not within same VPC.
- ✓
Modify the security group for the RDS database to allow inbound traffic from the EC2 instance's security group on port 3306.
Why this is correct
Security groups act as a firewall for instances.
- ✗
Add a NAT Gateway in a public subnet and route traffic through it.
Why it's wrong here
Unnecessary for internal VPC communication.
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Same concept, more angles
1 more way this is tested on ANS-C01
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A company has a VPC with a CIDR of 10.0.0.0/16. They have two subnets: subnet A (10.0.1.0/24) and subnet B (10.0.2.0/24). They launch an EC2 instance in subnet A and another in subnet B. The security groups for both instances allow all traffic from the other instance's private IP. However, the instances cannot communicate. What is the most likely cause?
hard- A.The instances are in different Availability Zones
- ✓ B.The network ACLs are blocking the traffic
- C.The security groups are stateful and block return traffic
- D.The route tables do not have a local route
Why B: The most likely cause is that the network ACLs (NACLs) are blocking the traffic. Security groups are stateful and allow return traffic automatically, but NACLs are stateless and require explicit inbound and outbound rules for both directions. By default, custom NACLs deny all traffic, so even though the security groups permit communication, the NACLs on subnets A and B must allow the traffic. The correct answer is B.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This ANS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the ANS-C01 exam.