The answer is to add inbound rules on the server security group allowing HTTP and HTTPS traffic from the client security group. This is correct because security groups are stateful, meaning that when you allow inbound traffic, the corresponding outbound response traffic is automatically permitted, regardless of any outbound rules. For EC2 instance communication, you only need to define the inbound rule on the receiving side; the return packets flow back without an explicit outbound rule. On the AWS Certified SysOps Administrator Associate SOA-C02 exam, this concept frequently appears in troubleshooting scenarios where connectivity fails despite permissive outbound rules, testing your understanding of stateful versus stateless firewalls. A common trap is adding unnecessary outbound rules on the client or inbound rules on the client, which are not required. Remember the memory tip: "Inbound in, outbound out—stateful lets the response out without a shout."
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 EC2 instances in the same VPC but different subnets. The instances can communicate over private IP addresses when security groups are set to allow all traffic, but fail when security groups are configured with specific rules. The Administrator wants to allow HTTP (port 80) and HTTPS (port 443) traffic from the client instance to the server instance. What security group rules are needed?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Add inbound rules on the server to allow HTTP and HTTPS from the client security group.
Option D is correct because security groups are stateful; you only need inbound rules on the server to allow HTTP/HTTPS from the client. The outbound response is automatically allowed. Option A is wrong because outbound rules on the client are not needed for response. Option B is wrong because inbound rules on the client are not needed. Option C is wrong because outbound rules on the server are not needed for response.
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.
✓
Add inbound rules on the server to allow HTTP and HTTPS from the client security group.
Why this is correct
Security groups are stateful; inbound rules on the server allow the request, and the response is automatically allowed.
Only the server needs inbound rules; the client does not need inbound rules for this traffic.
✗
Add outbound rules on both the client and server.
Why it's wrong here
Outbound rules on the server are not needed; the client needs outbound rules to send traffic.
✗
Add inbound rules on the client and outbound rules on the server.
Why it's wrong here
Inbound rules on the client are unnecessary; outbound rules on the server are not needed due to statefulness.
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: Add inbound rules on the server to allow HTTP and HTTPS from the client security group. — Option D is correct because security groups are stateful; you only need inbound rules on the server to allow HTTP/HTTPS from the client. The outbound response is automatically allowed. Option A is wrong because outbound rules on the client are not needed for response. Option B is wrong because inbound rules on the client are not needed. Option C is wrong because outbound rules on the server are not needed for response.
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.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Question Discussion
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