The correct answer is to configure a network security group (NSG) on the Application Gateway subnet to allow traffic only from Azure Front Door’s documented backend IP ranges. This works because Front Door publishes a specific set of backend IP prefixes that you can use in an NSG rule to create a strict allowlist, ensuring only validated Front Door traffic reaches the Application Gateway. On the AZ-500 exam, this scenario tests your understanding of network segmentation and service-specific access control, often appearing as a trap where candidates mistakenly rely on Front Door’s service tag (AzureFrontDoor.Backend) or the Application Gateway’s frontend IP, both of which are less reliable—the service tag may not cover all backend scenarios, and the gateway’s IP is not static. A common memory tip is to think of the NSG as a bouncer checking a VIP list: you must use the published IP ranges (the list) rather than a generic tag or a changing address. Remember: for Front Door + Application Gateway, always lock down the subnet with the documented backend IPs, not the service tag.
AZ-500 Secure networking Practice Question
This AZ-500 practice question tests your understanding of secure networking. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
Your company uses Azure Front Door to globally distribute traffic to a web app. You need to ensure that only traffic from Front Door can reach the web app, and all other traffic is blocked. The web app is behind an Azure Application Gateway. What is the most secure and reliable configuration?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Configure an NSG on the Application Gateway subnet to allow traffic only from the Front Door backend IP ranges.
Option D is correct because Front Door's backend IP ranges are documented and can be used in an NSG to allow only Front Door traffic. Option A is wrong because the Application Gateway's frontend IP is not static and can change. Option B is wrong because Front Door's service tag is AzureFrontDoor.Backend, but the question requires using the Application Gateway, and the service tag might not be sufficient for the application gateway's backend. Option C is wrong because private endpoints are for PaaS, not for Application Gateway.
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.
✗
Place the web app behind a private endpoint and connect Front Door via private link.
Why it's wrong here
Private endpoint not applicable to Application Gateway.
✗
Use an NSG on the Application Gateway subnet with a deny-all rule and allow the AzureFrontDoor.Backend service tag.
Why it's wrong here
The service tag is for Front Door's backend, not frontend.
✓
Configure an NSG on the Application Gateway subnet to allow traffic only from the Front Door backend IP ranges.
Why this is correct
Front Door publishes backend IP ranges that can be used in NSGs.
Restrict the Application Gateway to only accept traffic from the Front Door frontend IP.
Why it's wrong here
Front Door frontend IP is anycast and not static.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this AZ-500 question in full detail.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related AZ-500 subnetting questions on CIDR, address ranges, and subnet selection.
Secure networking — This question tests Secure networking — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Configure an NSG on the Application Gateway subnet to allow traffic only from the Front Door backend IP ranges. — Option D is correct because Front Door's backend IP ranges are documented and can be used in an NSG to allow only Front Door traffic. Option A is wrong because the Application Gateway's frontend IP is not static and can change. Option B is wrong because Front Door's service tag is AzureFrontDoor.Backend, but the question requires using the Application Gateway, and the service tag might not be sufficient for the application gateway's backend. Option C is wrong because private endpoints are for PaaS, not for Application Gateway.
What should I do if I get this AZ-500 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 AZ-500 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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