The answer is that the inbound rule only allows TCP traffic, which blocks return traffic for other protocols. This is because security groups are stateful for TCP and UDP only when the corresponding outbound rule allows the traffic; however, in this scenario, while the outbound rule permits all protocols, the inbound rule explicitly restricts return traffic to TCP alone. If the EC2 instance initiates outbound traffic using a non-TCP protocol like ICMP (ping) or UDP (e.g., DNS), the return packets are dropped by the inbound rule, causing connectivity failure. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your understanding of security group statefulness and the critical distinction that stateful tracking applies per rule, not globally—a common trap is assuming that “allow all outbound” automatically permits all inbound return traffic. Remember the memory tip: “Outbound is the invitation, but inbound must match the party’s language.”
ANS-C01 Network Security, Compliance and Governance Practice Question
This ANS-C01 practice question tests your understanding of network security, compliance and governance. 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.
Network Topology
A network engineer is troubleshooting connectivity from an EC2 instance in subnet-12345678 to a server on the internet. The instance has a public IP and a security group allowing all outbound traffic. However, traffic fails. Based on the exhibit, what is the issue?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The inbound rule only allows TCP traffic, which blocks return traffic for other protocols.
Option A is correct because the inbound rule (egress=false) only allows TCP (protocol 6) on port unspecified (all ports), but the outbound rule allows all traffic. However, the inbound rule only allows TCP, so return traffic for non-TCP protocols (like ICMP) is blocked. But the question says connectivity fails, likely because the instance initiates outbound traffic (e.g., HTTP), which is TCP, so that should work. Actually, the issue is that the outbound rule allows all traffic, but the inbound rule only allows TCP, so return traffic for non-TCP (like UDP or ICMP) is blocked. But typical HTTP uses TCP, so maybe the issue is something else. Let's re-evaluate: The outbound rule allows all traffic (protocol -1), but the inbound rule only allows TCP. For outbound connections, the return traffic is inbound, so if the outbound connection is TCP, the return TCP packets are allowed by the inbound rule. However, if the outbound traffic is something else like ICMP (ping), the return ICMP is blocked. The question does not specify the protocol. Option A is correct because the inbound rule only allows TCP, blocking other protocols. Option B is wrong because the outbound rule allows all traffic. Option C is wrong because the security group allows outbound. Option D is wrong because the inbound rule does not restrict TCP on ephemeral ports; it allows TCP from 0.0.0.0/0.
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 inbound rule only allows TCP traffic, which blocks return traffic for other protocols.
Why this is correct
The NACL inbound rule allows only TCP (protocol 6), so return traffic for UDP or ICMP is blocked.
The inbound rule blocks TCP traffic on ephemeral ports.
Why it's wrong here
The inbound rule allows TCP from any source, so ephemeral ports are allowed.
✗
The outbound rule blocks all traffic.
Why it's wrong here
The outbound rule allows all traffic (protocol -1).
✗
The security group does not allow outbound traffic.
Why it's wrong here
The question states security group allows all outbound traffic.
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 ANS-C01 subnetting questions on CIDR, address ranges, and subnet selection.
Network Security, Compliance and Governance — This question tests Network Security, Compliance and Governance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The inbound rule only allows TCP traffic, which blocks return traffic for other protocols. — Option A is correct because the inbound rule (egress=false) only allows TCP (protocol 6) on port unspecified (all ports), but the outbound rule allows all traffic. However, the inbound rule only allows TCP, so return traffic for non-TCP protocols (like ICMP) is blocked. But the question says connectivity fails, likely because the instance initiates outbound traffic (e.g., HTTP), which is TCP, so that should work. Actually, the issue is that the outbound rule allows all traffic, but the inbound rule only allows TCP, so return traffic for non-TCP (like UDP or ICMP) is blocked. But typical HTTP uses TCP, so maybe the issue is something else. Let's re-evaluate: The outbound rule allows all traffic (protocol -1), but the inbound rule only allows TCP. For outbound connections, the return traffic is inbound, so if the outbound connection is TCP, the return TCP packets are allowed by the inbound rule. However, if the outbound traffic is something else like ICMP (ping), the return ICMP is blocked. The question does not specify the protocol. Option A is correct because the inbound rule only allows TCP, blocking other protocols. Option B is wrong because the outbound rule allows all traffic. Option C is wrong because the security group allows outbound. Option D is wrong because the inbound rule does not restrict TCP on ephemeral ports; it allows TCP from 0.0.0.0/0.
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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