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Question

Difficulty: MediumCreate and Configure Virtual Networks and Subnets

An administrator is planning the deployment of a new virtual network named `vnet-prod` to support a web application and secure administrative access. The virtual network is assigned the address space 10.200.0.0/2210.200.0.0/22.

The administrator must configure three subnets within `vnet-prod` to meet the following requirements:
- A subnet for the web application servers that must support up to 5858 concurrent virtual machines.
- A subnet for Azure Bastion to allow secure administration.
- A subnet for a database tier that must support up to 44 virtual machines.

The administrator wants to minimize IP address space waste and allocate the smallest possible CIDR blocks that satisfy these requirements.

Which of the following subnet configurations should the administrator use?

  1. AzureBastionSubnet: 10.200.0.0/2610.200.0.0/26; Subnet-Web: 10.200.0.64/2610.200.0.64/26; Subnet-DB: 10.200.0.128/2810.200.0.128/28Answer
  2. B
    BastionSubnet: 10.200.0.0/2610.200.0.0/26; Subnet-Web: 10.200.0.64/2610.200.0.64/26; Subnet-DB: 10.200.0.128/2810.200.0.128/28
  3. C
    AzureBastionSubnet: 10.200.0.0/2710.200.0.0/27; Subnet-Web: 10.200.0.32/2610.200.0.32/26; Subnet-DB: 10.200.0.96/2810.200.0.96/28
  4. D
    AzureBastionSubnet: 10.200.0.0/2610.200.0.0/26; Subnet-Web: 10.200.0.64/2610.200.0.64/26; Subnet-DB: 10.200.0.128/2910.200.0.128/29

Answer

AzureBastionSubnet: 10.200.0.0/26; Subnet-Web: 10.200.0.64/26; Subnet-DB: 10.200.0.128/28
The configuration specifying AzureBastionSubnet at 10.200.0.0/2610.200.0.0/26, Subnet-Web at 10.200.0.64/2610.200.0.64/26, and Subnet-DB at 10.200.0.128/2810.200.0.128/28 is correct because it satisfies all naming, sizing, and usability constraints: first, the Azure Bastion subnet is named exactly `AzureBastionSubnet` and is sized at `/26` (the minimum supported size); second, the web subnet is sized at `/26` (providing 64−5=5964 - 5 = 59 usable IPs), which is the smallest CIDR block that can accommodate 5858 virtual machines; third, the database subnet is sized at `/28` (providing 16−5=1116 - 5 = 11 usable IPs), which is the smallest CIDR block that can accommodate 44 virtual machines since a `/29` would only provide 33 usable IPs.

Step-by-Step Solution

1
Determine the naming and size requirements for the Azure Bastion subnet.
The subnet must be named exactly `AzureBastionSubnet` and have a prefix size of `/26` or larger.
Azure requires this specific configuration to deploy and scale the Azure Bastion resource.
2
Calculate the minimum subnet size for the web application servers.
A `/26` subnet is required.
The web tier requires 5858 VM IPs plus 55 Azure-reserved IPs, totaling 6363 IP addresses. A `/26` subnet provides 6464 IP addresses (5959 usable), whereas a `/27` subnet only provides 3232 IP addresses (2727 usable).
3
Calculate the minimum subnet size for the database tier.
A `/28` subnet is required.
The database tier requires 44 VM IPs plus 55 Azure-reserved IPs, totaling 99 IP addresses. A `/28` subnet provides 1616 IP addresses (1111 usable), whereas a `/29` subnet only provides 88 IP addresses (33 usable).

Key Concept

Azure Virtual Network subnets reserve 5 IP addresses (the first four and the last one). Therefore, the capacity calculation must always be: Usable IPs = Total IPs - 5. Furthermore, specialized subnets like Azure Bastion have strict minimum sizes (/26) and naming requirements.
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Topics

Create and Configure Virtual Networks and Subnets