The answer is to check the EC2 instance’s operating system firewall, such as iptables or Windows Firewall, because a host-based firewall can block port 443 even when ICMP (ping) succeeds over Direct Connect. This is a classic case where “direct connect ping works but port blocked” reveals that the network path is up at Layer 3, but a higher-layer filter on the instance itself is denying TCP traffic on the specific port. On the AWS Certified Advanced Networking Specialty ANS-C01 exam, this question tests your ability to isolate host-level issues from network-level configurations; a common trap is to assume the security group or route tables are at fault when they already permit the traffic. Remember that ping uses ICMP, which is often allowed by default, while HTTPS requires explicit TCP port 443 permission at the OS firewall. Memory tip: “Ping is Layer 3, port is Layer 4—check the host’s own door.”
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 uses AWS Direct Connect with a private VIF to connect its data center to a VPC. The VPC has multiple subnets. The on-premises network team reports that they can ping the VPC's private IP addresses but cannot connect to an EC2 instance's port 443. The EC2 instance's security group allows HTTPS from the on-premises CIDR. What should the engineer check NEXT?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Check the EC2 instance's operating system firewall (e.g., iptables, Windows Firewall) to see if port 443 is blocked.
Option C is correct because the instance may have a host-based firewall blocking the port; this is a common issue. Option A is wrong because route tables are for routing, not port filtering. Option B is wrong because security group already allows the traffic. Option D is wrong because the virtual interface is up (ping works).
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.
✗
Check the security group's inbound rules for port 443.
Why it's wrong here
Already checked and allowed.
✗
Confirm that the Direct Connect virtual interface is in the 'up' state.
Why it's wrong here
Ping works, so the interface is up.
✓
Check the EC2 instance's operating system firewall (e.g., iptables, Windows Firewall) to see if port 443 is blocked.
Why this is correct
OS firewall can block even if security group allows.
Verify that the route tables in the VPC have a route back to the on-premises CIDR.
Why it's wrong here
Ping works, so routing is fine.
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: Check the EC2 instance's operating system firewall (e.g., iptables, Windows Firewall) to see if port 443 is blocked. — Option C is correct because the instance may have a host-based firewall blocking the port; this is a common issue. Option A is wrong because route tables are for routing, not port filtering. Option B is wrong because security group already allows the traffic. Option D is wrong because the virtual interface is up (ping works).
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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