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Difficulty: EasyCreate and Configure Virtual Networks and Subnets

An administrator is configuring the network topology for a resource group named `RG-Core` in an Azure subscription. The resource group contains a virtual network named `VNet-Core`. The administrator must add a new subnet named `Subnet-Workloads` to `VNet-Core` to host exactly 1212 virtual machines, with each requiring a dedicated private IP address. No other resources will be placed in this subnet. To minimize IP address allocation, the subnet must use the smallest possible prefix that accommodates the virtual machines. Which subnet mask should the administrator choose?

  1. A
    /28/28
  2. /27/27Answer
  3. C
    /29/29
  4. D
    /26/26

Answer

The /27/27 subnet mask is the smallest subnet that can accommodate the virtual machines, providing 2727 usable IP addresses out of 3232 total IP addresses after accounting for the 55 IP addresses reserved by Azure.
The /27/27 subnet mask is correct because a /27/27 block provides 3232 total IP addresses. Since Azure reserves 55 IP addresses in every subnet (the first 44 and the last 11 address), this leaves 32−5=2732 - 5 = 27 usable IP addresses. This is sufficient for the 1212 virtual machines and is the smallest available subnet prefix that meets the requirement.

Step-by-Step Solution

1
Calculate the total number of private IP addresses required by the virtual machines.
1212 IP addresses are required for the 1212 virtual machines.
Each virtual machine requires a single private IP address.
2
Add the number of IP addresses reserved by Azure in every subnet.
12+5=1712 + 5 = 17 total IP addresses are required.
Azure reserves 55 IP addresses in every subnet (the first 44 and the last 11 address).
3
Determine the smallest CIDR block that provides at least the required total number of IP addresses.
A /27/27 subnet mask provides 232−27=322^{32-27} = 32 total IP addresses (2727 usable), whereas a /28/28 subnet mask only provides 232−28=162^{32-28} = 16 total IP addresses (1111 usable).
The /28/28 subnet mask is too small (11<1211 < 12), so the next larger size, /27/27, must be used to minimize address space wastage while meeting the requirement.

Key Concept

Azure reserves 55 IP addresses in each subnet, which must be accounted for when planning subnet sizes.
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Topics

Create and Configure Virtual Networks and Subnets