The answer is that the web server’s security group in VPC B lacks an inbound rule allowing TCP port 443 from VPC A’s CIDR. This is the most likely cause because ICMP (ping) succeeds while TCP fails, indicating a stateful security group is selectively blocking the TCP handshake; security groups evaluate inbound traffic separately from outbound, and ICMP is permitted while TCP 443 is not. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this scenario tests your understanding that VPC peering is a one-to-one, non-transitive connection, and that security groups are stateful—so a missing rule for a specific protocol and port will block only that traffic, unlike NACLs which are stateless and would block both ICMP and TCP equally. A common trap is assuming NACLs are at fault when ping works, but remember: if ICMP passes but TCP fails, the culprit is almost always a security group rule, not a NACL. Memory tip: “Ping works, TCP fails? Check the SG—ICMP is allowed, but your port’s in jail.”
ANS-C01 Network Design Practice Question
This ANS-C01 practice question tests your understanding of network design. 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 peering connection between VPC A (10.0.0.0/16) and VPC B (10.1.0.0/16). Both VPCs have subnets with EC2 instances. The security groups allow all traffic between the instances. The instances in VPC A can ping the instances in VPC B, but cannot initiate TCP connections to a web server running on port 443 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 security group for the web server in VPC B does not allow inbound traffic from VPC A on TCP port 443.
Option D is correct. VPC peering does not support transitive routing, so if there is a NACL or security group rule that allows ICMP but not TCP, or if the web server's security group only allows traffic from specific sources, the issue could be security group rules. However, the most common cause is that the security group for the web server in VPC B does not allow inbound TCP 443 from VPC A CIDR. Option A is incorrect because VPC peering supports TCP. Option B is incorrect because DNS resolution is not required for TCP. Option C is incorrect because NACLs are stateless and would affect both ICMP and TCP if misconfigured.
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 peering connection does not support TCP traffic.
Why it's wrong here
VPC peering supports all protocols.
✗
The DNS resolution settings for the VPC peering are not enabled.
Why it's wrong here
DNS resolution is for hostname resolution, not TCP connectivity.
✓
The security group for the web server in VPC B does not allow inbound traffic from VPC A on TCP port 443.
Why this is correct
Security groups are stateful; if inbound rule missing, TCP connections are denied while ICMP might be allowed.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The network ACL in VPC B is blocking inbound TCP traffic on port 443.
Why it's wrong here
If NACL blocked TCP, ICMP might also be affected if not explicitly allowed; but ICMP works, so unlikely.
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 Design — This question tests Network Design — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The security group for the web server in VPC B does not allow inbound traffic from VPC A on TCP port 443. — Option D is correct. VPC peering does not support transitive routing, so if there is a NACL or security group rule that allows ICMP but not TCP, or if the web server's security group only allows traffic from specific sources, the issue could be security group rules. However, the most common cause is that the security group for the web server in VPC B does not allow inbound TCP 443 from VPC A CIDR. Option A is incorrect because VPC peering supports TCP. Option B is incorrect because DNS resolution is not required for TCP. Option C is incorrect because NACLs are stateless and would affect both ICMP and TCP if misconfigured.
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.
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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