The answer is that the on-premises network does not have a route back to the VPC CIDR through the Direct Connect. This is because for a ping to succeed, traffic must flow in both directions; while the on-premises servers can reach the EC2 instances, the return traffic from the EC2 instances is likely taking an internet path instead of the Direct Connect return route if the on-premises side lacks a specific route pointing the VPC CIDR back to the Direct Connect interface. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this scenario tests your understanding of asymmetric routing and the fact that Direct Connect requires route propagation and static routes on both ends—a common trap is assuming that a working ping from on-premises means the connection is fully bidirectional. Remember the memory tip: “Ping is a round trip, not a one-way street—check both route tables.”
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 uses AWS Direct Connect to connect its on-premises data center to a VPC. The VPC has a private subnet with EC2 instances that need to communicate with on-premises servers. The on-premises network team reports that they can ping the EC2 instances, but the EC2 instances cannot ping the on-premises servers. The SysOps administrator checks the route tables and finds that the VPC has a route to the on-premises CIDR via the virtual private gateway. The security groups allow all ICMP traffic. 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 on-premises network does not have a route back to the VPC CIDR through the Direct Connect
Option C is correct because the on-premises servers need to have a route back to the VPC CIDR through the Direct Connect. The fact that they can ping the EC2 instances indicates that traffic from on-premises to VPC is working, but the return traffic may be going through the internet instead of Direct Connect if the on-premises route is not configured. Option A is wrong because the VPC route is present. Option B is wrong because security groups allow ICMP. Option D is wrong because NACLs default to allow 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 security group on the EC2 instances blocks outbound ICMP
Why it's wrong here
Security groups allow all outbound by default.
✗
The network ACL for the private subnet blocks outbound traffic
Why it's wrong here
Default NACLs allow all outbound.
✓
The on-premises network does not have a route back to the VPC CIDR through the Direct Connect
Why this is correct
For two-way communication, both sides must have routes via Direct Connect.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The VPC route table does not have a route to the on-premises CIDR via the virtual private gateway
Why it's wrong here
The administrator already verified the route exists.
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 on-premises network does not have a route back to the VPC CIDR through the Direct Connect — Option C is correct because the on-premises servers need to have a route back to the VPC CIDR through the Direct Connect. The fact that they can ping the EC2 instances indicates that traffic from on-premises to VPC is working, but the return traffic may be going through the internet instead of Direct Connect if the on-premises route is not configured. Option A is wrong because the VPC route is present. Option B is wrong because security groups allow ICMP. Option D is wrong because NACLs default to allow 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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