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 SysOps administrator is troubleshooting connectivity issues between two VPCs in different AWS Regions. Both VPCs are connected via a VPC Peering connection. The route tables in both VPCs have routes pointing to the peering connection. Security groups allow all traffic. However, an EC2 instance in VPC A cannot ping an EC2 instance in VPC B. 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 route tables in the subnets where the instances reside do not include a route to the peered VPC's CIDR.
Option D is correct because VPC Peering does not support transitive routing; each VPC must have explicit routes to the other VPC's CIDR. If the route tables are correctly configured, the issue is likely that the instances do not have the other VPC's CIDR in their route tables. Option A is incorrect because VPC Peering works across regions. Option B is incorrect because the security groups can reference each other if the peered VPC's CIDR is added, but not by security group ID across regions. Option C is incorrect because NACLs are stateless and need rules for both directions.
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 network ACLs in subnets are blocking ICMP traffic.
Why it's wrong here
While possible, this is less likely if security groups allow all traffic; NACLs must explicitly allow ICMP.
✗
VPC Peering is not supported across AWS Regions.
Why it's wrong here
Inter-Region VPC Peering is supported.
✓
The route tables in the subnets where the instances reside do not include a route to the peered VPC's CIDR.
Why this is correct
Subnet route tables must have explicit routes for the peered VPC's CIDR to the peering connection.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The security groups do not allow traffic from the peered VPC's security group ID.
Why it's wrong here
Security groups can reference security group IDs only within the same VPC; cross-peering requires CIDR-based rules.
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 route tables in the subnets where the instances reside do not include a route to the peered VPC's CIDR. — Option D is correct because VPC Peering does not support transitive routing; each VPC must have explicit routes to the other VPC's CIDR. If the route tables are correctly configured, the issue is likely that the instances do not have the other VPC's CIDR in their route tables. Option A is incorrect because VPC Peering works across regions. Option B is incorrect because the security groups can reference each other if the peered VPC's CIDR is added, but not by security group ID across regions. Option C is incorrect because NACLs are stateless and need rules for both directions.
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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