- A
Communication is not allowed by default; a VPC peering connection is required
Why wrong: VPC peering is for different VPCs.
- B
Communication is allowed only if they are in the same subnet
Why wrong: Cross-subnet communication is allowed by default.
- C
Communication is allowed only if a transit gateway is attached
Why wrong: A transit gateway is not needed for intra-VPC communication.
- D
Communication is allowed by default because they are in the same VPC
Instances in the same VPC can communicate by default.
Question 568 of 1,705
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often 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.
Detailed technical explanation
How to think about this question
Under the hood, each VPC has an implicit router that maintains a local route for the VPC's CIDR block, enabling direct communication between all subnets. The default network ACL allows all inbound and outbound traffic, and the default security group allows all outbound traffic and inbound traffic from other instances in the same security group. In a real-world scenario, if you need to restrict inter-subnet traffic, you would modify network ACLs or security group rules, but by default, no such restrictions exist.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
TExam Day Tips
- Watch for words such as best, first, most likely and least administrative effort.
- Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A healthcare organisation deploys an application with a public-facing web tier and a private database tier. The database subnet has no public IP and only accepts connections from the web tier's security group. Questions like this test whether you can design cloud network isolation using VNets/VPCs, subnets, and security group rules.
Visual reference
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FAQ
Questions learners often ask
What does this ANS-C01 question test?
Network Design — This question tests Network Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this ANS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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