The answer is that the VPC does not have DNS resolution enabled, which prevents the EC2 instance from resolving the RDS endpoint hostname. For VPC DNS resolution required for RDS connectivity, the RDS database is assigned a DNS name that must be translated to an IP address; without the VPC’s DNS resolution attribute set to true, the EC2 instance cannot resolve that name, even if security groups and network ACLs are correctly configured. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of how DNS settings in a VPC directly impact service connectivity, especially when an RDS instance resides in a private subnet without internet access. A common trap is to blame security groups or route tables, but the core issue is often the missing DNS resolution flag. Remember the mnemonic: “No DNS, no address” — if the VPC can’t resolve names, your EC2 can’t find RDS.
SOA-C02 Networking and Content Delivery Practice Question
This SOA-C02 practice question tests your understanding of networking and content delivery. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 multiple subnets. An EC2 instance in a public subnet needs to communicate with an RDS database in a private subnet. The RDS security group allows inbound traffic from the EC2 instance's security group. However, the EC2 instance cannot connect. What is the most likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The VPC does not have DNS resolution enabled, so the RDS endpoint cannot be resolved.
Option C is correct because if the RDS is in a private subnet, it may not have a route to the internet or to the public subnet, but the issue is likely that the VPC does not have DNS resolution enabled, causing name resolution failure. Option A is wrong because the security group is already configured. Option B is wrong because the instance has a public IP. Option D is wrong because NACLs are stateless but default allows all.
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.
✓
The VPC does not have DNS resolution enabled, so the RDS endpoint cannot be resolved.
Why this is correct
Correct because RDS endpoints are DNS names; without DNS resolution, the instance cannot connect.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The network ACL for the private subnet blocks inbound traffic from the public subnet.
Why it's wrong here
Incorrect because default NACLs allow all traffic.
✗
The security group of the RDS database does not allow outbound traffic.
Why it's wrong here
Incorrect because security groups are stateful; outbound is not needed for inbound replies.
✗
The EC2 instance does not have a public IP address.
Why it's wrong here
Incorrect because the instance is in a public subnet and likely has a public IP, but internal communication uses private IPs.
Common exam traps
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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Networking and Content Delivery — This question tests Networking and Content Delivery — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The VPC does not have DNS resolution enabled, so the RDS endpoint cannot be resolved. — Option C is correct because if the RDS is in a private subnet, it may not have a route to the internet or to the public subnet, but the issue is likely that the VPC does not have DNS resolution enabled, causing name resolution failure. Option A is wrong because the security group is already configured. Option B is wrong because the instance has a public IP. Option D is wrong because NACLs are stateless but default allows all.
What should I do if I get this SOA-C02 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 SOA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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