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Difficulty: EasyCreate and Configure Virtual Networks and Subnets

Your company has an Azure subscription containing a resource group named `RG-Operations` and a virtual network named `VNet-Core` with an address space of 172.16.0.0/12172.16.0.0/12. You need to create a new subnet named `Subnet-App` to deploy 28 virtual machines that require individual private IP addresses. What is the smallest subnet size (CIDR prefix) you can configure for `Subnet-App` to support these virtual machines?

  1. A
    `/27`
  2. `/26`Answer
  3. C
    `/28`
  4. D
    `/25`

Answer

The smallest subnet size (CIDR prefix) that can support 28 virtual machines is `/26`.
To host 28 virtual machines, you need 28 IP addresses. Since Azure reserves 5 IP addresses in every subnet, the total number of IP addresses required is 28+5=3328 + 5 = 33. A prefix of `/27` only yields 3232 total IP addresses (27 usable), which is one address short. Therefore, the next larger size, which is `/26` (providing 6464 total addresses and 59 usable addresses), is the smallest prefix size that can support the requirements.

Step-by-Step Solution

1
Identify the host requirement and the Azure subnet reservation rule.
28 virtual machines are required, and Azure reserves 5 IP addresses per subnet (the first 4 and the last 1).
To find the minimum address space, we must sum the required host IP addresses and Azure's reserved IP addresses.
2
Calculate the total minimum number of IP addresses required.
28 hosts+5 reserved IPs=33 IP addresses28 \text{ hosts} + 5 \text{ reserved IPs} = 33 \text{ IP addresses}.
Any subnet size chosen must provide at least 33 total IP addresses.
3
Determine the smallest CIDR block that provides at least 33 IP addresses.
A `/27` subnet provides 232−27=322^{32-27} = 32 IP addresses (too small). A `/26` subnet provides 232−26=642^{32-26} = 64 IP addresses.
A `/26` subnet is the smallest standard block that can accommodate the required 33 IP addresses.

Key Concept

Subnet sizing and Azure reserved IP addresses
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Topics

Create and Configure Virtual Networks and Subnets