ANS-C01 Network Design Practice Question
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?
⚠ Common exam trap
Watch out — candidates often assume security groups are the only firewall layer or forget that NACLs are stateless and require explicit rules for both directions, leading them to overlook NACL misconfigurations when security groups appear permissive.
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 ACLs are blocking the traffic
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.
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 instances are in different Availability Zones
Why it's wrong here
Instances in same VPC can communicate across AZs.
- ✓
The network ACLs are blocking the traffic
Why this is correct
NACLs are stateless and need to allow both outbound and inbound traffic for each direction.
- ✗
The security groups are stateful and block return traffic
Why it's wrong here
Security groups are stateful, automatically allow return traffic.
- ✗
The route tables do not have a local route
Why it's wrong here
VPC automatically adds local route.
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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.