The answer is that communication is allowed by default with no additional configuration needed. This is because all subnets within a single VPC are inherently connected via the VPC’s implicit router, which enables routing between any two subnets without requiring a VPC peering, transit gateway, or custom route table entries. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this concept tests your understanding of the VPC’s default networking behavior, often appearing as a trick question where candidates assume subnets are isolated by default. A common trap is confusing subnet isolation with the need for a NAT gateway or internet gateway—neither is required for internal subnet-to-subnet traffic. Remember the memory tip: “Same VPC, same network; no extra steps, just security checks.” The only barriers to this default communication are security groups and network ACLs, which must explicitly allow the traffic.
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 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, instances within the same VPC can communicate with each other regardless of subnet, as long as security groups and network ACLs allow traffic. No additional configuration is needed.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Design — This question tests Network Design — CIDR notation defines the prefix length..
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, instances within the same VPC can communicate with each other regardless of subnet, as long as security groups and network ACLs allow traffic. No additional configuration is needed.
What should I do if I get this ANS-C01 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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