ANS-C01 Network Design Practice Question
A company has a VPC with 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 EC2 instance in subnet B. The instances are in the same VPC. What is the default behavior?
⚠ Common exam trap
Many candidates confuse subnet isolation with VPC isolation, mistakenly thinking that different subnets require a peering or gateway to communicate, when in fact the VPC's internal routing handles it automatically.
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
✓
Communication is allowed by default because they are in the same VPC
By default, all instances within the same VPC can communicate with each other regardless of which subnet they reside in, as long as the VPC's main route table and network ACLs allow the traffic. The VPC's implicit router provides connectivity between all subnets, and the default security group allows inbound traffic from other members of the same security group. Therefore, an EC2 instance in subnet A can communicate with an instance in subnet B without any additional configuration.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Communication is not allowed by default; a VPC peering connection is required
Why it's wrong here
VPC peering is for different VPCs.
- ✗
Communication is allowed only if they are in the same subnet
Why it's wrong here
Cross-subnet communication is allowed by default.
- ✗
Communication is allowed only if a transit gateway is attached
Why it's wrong here
A transit gateway is not needed for intra-VPC communication.
- ✓
Communication is allowed by default because they are in the same VPC
Why this is correct
Instances in the same VPC can communicate by default.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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