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Design security solutions for infrastructure →mediumMultiple ChoiceObjective-mapped

SC-100 Design security solutions for infrastructure Practice Question

This SC-100 practice question tests your understanding of design security solutions for infrastructure. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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.

Exhibit

{
  "type": "Microsoft.Network/networkSecurityGroups",
  "apiVersion": "2023-09-01",
  "name": "nsg-web",
  "properties": {
    "securityRules": [
      {
        "name": "AllowHTTP",
        "properties": {
          "protocol": "Tcp",
          "sourcePortRange": "*",
          "destinationPortRange": "80",
          "sourceAddressPrefix": "Internet",
          "destinationAddressPrefix": "*",
          "access": "Allow",
          "priority": 100,
          "direction": "Inbound"
        }
      },
      {
        "name": "DenyAllOther",
        "properties": {
          "protocol": "*",
          "sourcePortRange": "*",
          "destinationPortRange": "*",
          "sourceAddressPrefix": "*",
          "destinationAddressPrefix": "*",
          "access": "Deny",
          "priority": 1000,
          "direction": "Inbound"
        }
      }
    ]
  }
}

Refer to the exhibit. The NSG is applied to a subnet containing a web server. The web server is not receiving HTTP traffic. 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.

Question 1mediummultiple choice
Review the full subnetting walkthrough →

Exhibit

{
  "type": "Microsoft.Network/networkSecurityGroups",
  "apiVersion": "2023-09-01",
  "name": "nsg-web",
  "properties": {
    "securityRules": [
      {
        "name": "AllowHTTP",
        "properties": {
          "protocol": "Tcp",
          "sourcePortRange": "*",
          "destinationPortRange": "80",
          "sourceAddressPrefix": "Internet",
          "destinationAddressPrefix": "*",
          "access": "Allow",
          "priority": 100,
          "direction": "Inbound"
        }
      },
      {
        "name": "DenyAllOther",
        "properties": {
          "protocol": "*",
          "sourcePortRange": "*",
          "destinationPortRange": "*",
          "sourceAddressPrefix": "*",
          "destinationAddressPrefix": "*",
          "access": "Deny",
          "priority": 1000,
          "direction": "Inbound"
        }
      }
    ]
  }
}

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

The sourceAddressPrefix 'Internet' does not include all source IPs

Option B is correct because the rule uses 'Internet' as sourceAddressPrefix, which is a service tag that does not include all public IPs. The actual source IPs may not be covered. Option A is wrong because priority 100 is high enough (lower number = higher priority). Option C is wrong because the rules are correct for inbound traffic. Option D is wrong because the DenyAllOther rule has a higher priority number (1000) than AllowHTTP (100), so it is evaluated after AllowHTTP.

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 DenyAllOther rule has a lower priority than AllowHTTP

    Why it's wrong here

    DenyAllOther has priority 1000, which is lower than 100.

  • ✗

    The rule direction is Inbound, but the traffic is outbound

    Why it's wrong here

    HTTP request is inbound to the server.

  • ✓

    The sourceAddressPrefix 'Internet' does not include all source IPs

    Why this is correct

    'Internet' service tag may not cover all public IPs.

    Clue confirmation

    The clue word "most likely" in the question point toward this answer.

    Related concept

    CIDR notation defines the prefix length.

  • ✗

    The priority of the AllowHTTP rule is too low (100)

    Why it's wrong here

    Priority 100 is high; lower numbers are higher priority.

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 SC-100 subnetting questions on CIDR, address ranges, and subnet selection.

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FAQ

Questions learners often ask

What does this SC-100 question test?

Design security solutions for infrastructure — This question tests Design security solutions for infrastructure — CIDR notation defines the prefix length..

What is the correct answer to this question?

The correct answer is: The sourceAddressPrefix 'Internet' does not include all source IPs — Option B is correct because the rule uses 'Internet' as sourceAddressPrefix, which is a service tag that does not include all public IPs. The actual source IPs may not be covered. Option A is wrong because priority 100 is high enough (lower number = higher priority). Option C is wrong because the rules are correct for inbound traffic. Option D is wrong because the DenyAllOther rule has a higher priority number (1000) than AllowHTTP (100), so it is evaluated after AllowHTTP.

What should I do if I get this SC-100 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 SC-100 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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Last reviewed: Jun 21, 2026

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