Choosing the Right Subnet Prefix for 30 VM NICs in Azure
Exhibit
Planned VNet address space: 10.70.0.0/16 Subnet A requirement: about 30 VM NICs Subnet B requirement: about 8 VM NICs and one future jump box Azure reserves 5 IP addresses in each subnet
Based on the exhibit, which subnet prefix should be used for Subnet A so it can support about 30 VM NICs?
Quick Answer
The answer is a /26 subnet prefix. This is correct because a /26 subnet in Azure provides 64 total IP addresses, calculated as 2^(32-26), with 62 usable for VM NICs after Azure reserves five addresses for networking purposes. For your AZ-104 exam, this question tests your ability to match subnet size to workload requirements, specifically ensuring you account for Azure’s five reserved IPs (network, gateway, broadcast, and two internal DNS) when calculating usable addresses. A common trap is forgetting that Azure reserves more addresses than the traditional two, so a /27 (32 total, 27 usable) would be too tight for 30 NICs, while a /26 gives comfortable headroom for growth. The key takeaway: always subtract five from the total IP count to get usable Azure addresses. Memory tip: “/26 gives 64 total, 62 usable—plenty for 30 NICs plus room to expand.”
⚠ Common exam trap
The trap here is that candidates often forget Azure reserves five IP addresses per subnet, not just the network and broadcast addresses, leading them to incorrectly choose /27 thinking 32 total addresses are enough. In reality, a /27 subnet has only 27 usable addresses after Azure reservations, which cannot support about 30 VM NICs; /26 is required.
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
✓
/26
The correct option is C, /26, because a /26 prefix provides 2^(32-26) = 64 total addresses. Azure reserves 5 IP addresses per subnet, leaving 64 - 5 = 59 usable addresses, which comfortably supports about 30 VM NICs. A /28 (option A) provides only 16 total addresses (11 usable after Azure reservations), and a /27 (option B) provides 32 total addresses (27 usable after Azure reservations), which is too small for about 30 VM NICs. A /25 (option D) provides 128 total addresses (123 usable), which would also work but is larger than necessary for this requirement, making /26 the appropriate fit.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
/28
Why it's wrong here
A /28 subnet provides too few usable addresses after Azure reservations. It cannot support about 30 VM NICs.
When this WOULD be correct
This option would be correct if the question asked for a subnet to support about 10 VM NICs (e.g., 'support about 10 VM NICs'), as /28 provides 14 usable addresses, meeting that requirement.
- ✗
/27
Why it's wrong here
A /27 subnet has 32 total addresses, but Azure reserves 5 of them, leaving only 27 usable addresses. That is still short of the requirement for about 30 VM NICs.
When this WOULD be correct
This option would be correct if the question asked for a subnet to support about 25 VM NICs, or if the subnet only needed to support exactly 27 NICs with no additional overhead. For example, 'Which subnet prefix supports up to 27 VM NICs?'
- ✓
/26
Why this is correct
A /26 subnet is the smallest option listed that can support about 30 VM NICs after Azure reserves its five addresses. /26 provides 64 total addresses and enough usable IPs for the requirement, leaving a little room for growth. This is the most efficient choice among the options shown.
- ✗
/25
Why it's wrong here
A /25 subnet would also work, but it uses more address space than necessary. For an easy planning question, the smallest prefix that meets the need is the better answer.
When this WOULD be correct
This option would be correct in a scenario where Subnet A needs to support about 120 VM NICs (or a similar number requiring at least 126 addresses). For example, if the requirement were 'about 120 NICs', /25 would be the appropriate subnet prefix.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The AZ-104 exam frequently reuses these exact scenarios with slightly different constraints.
✓/26Correct answer▾
Why this is correct
A /26 subnet is the smallest option listed that can support about 30 VM NICs after Azure reserves its five addresses. /26 provides 64 total addresses and enough usable IPs for the requirement, leaving a little room for growth. This is the most efficient choice among the options shown.
✗/28Wrong answer — click to see why▾
Why this is wrong here
A /28 subnet provides only 14 usable IP addresses (2^(32-28)-2), which is insufficient for about 30 VM NICs. The question requires supporting approximately 30 NICs, so a /26 subnet with 62 usable addresses is the minimum correct choice.
★ When this WOULD be the correct answer
This option would be correct if the question asked for a subnet to support about 10 VM NICs (e.g., 'support about 10 VM NICs'), as /28 provides 14 usable addresses, meeting that requirement.
Why candidates choose this
Candidates may mistakenly think /28 is sufficient because they forget to subtract 2 for network and broadcast addresses, or they miscalculate the number of hosts (e.g., thinking 2^4 = 16 usable addresses).
✗/27Wrong answer — click to see why▾
Why this is wrong here
A /27 subnet provides only 30 usable IP addresses (32 total minus 5 reserved), which is insufficient for about 30 VM NICs plus other Azure reserved addresses (e.g., gateway, Azure DNS). The question requires support for about 30 VM NICs, so a /26 with 62 usable addresses is needed.
★ When this WOULD be the correct answer
This option would be correct if the question asked for a subnet to support about 25 VM NICs, or if the subnet only needed to support exactly 27 NICs with no additional overhead. For example, 'Which subnet prefix supports up to 27 VM NICs?'
Why candidates choose this
Candidates often miscalculate usable addresses by forgetting Azure reserves 5 IPs per subnet, or they assume /27 (32 total) is enough for 'about 30' without accounting for the reserved addresses.
✗/25Wrong answer — click to see why▾
Why this is wrong here
A /25 subnet provides 126 usable IP addresses, which far exceeds the requirement of about 30 VM NICs. Using /25 would waste IP address space, and the question asks for the prefix that 'should be used' to support about 30 NICs, implying the most efficient choice.
★ When this WOULD be the correct answer
This option would be correct in a scenario where Subnet A needs to support about 120 VM NICs (or a similar number requiring at least 126 addresses). For example, if the requirement were 'about 120 NICs', /25 would be the appropriate subnet prefix.
Why candidates choose this
Candidates might choose /25 because they think a larger subnet is safer or because they miscalculate the number of usable addresses (e.g., forgetting to subtract 5 for Azure reserved addresses).
Analysis generated from the official AZ-104blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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1 more way this is tested on AZ-104
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Variation 1. Based on the exhibit, which subnet prefix is the smallest that can support the planned resources in Azure?
medium- A./27
- ✓ B./26
- C./25
- D./24
Why B: The subnet prefix /26 provides 64 IP addresses per subnet (2^(32-26) = 64), of which 59 are usable after Azure reserves 5 addresses (first 4 and last 1). The planned resources require 30 IP addresses, so /26 is the smallest prefix that meets this requirement. A /27 would only provide 32 total addresses (27 usable), which is insufficient.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This AZ-104 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the AZ-104 exam.