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Difficulty: MediumCreate and Configure Virtual Networks and Subnets

An organization is designing a new Azure Virtual Network named `vnet-shared` with an address space of 10.150.0.0/2210.150.0.0/22 to support their multi-tier applications and secure administrative access. The virtual network must contain the following:

- A subnet named `Subnet-App` to host exactly 12 virtual machines.
- A subnet to host an Azure Bastion deployment to allow secure management of the virtual machines.
- A subnet to host a VPN gateway for hybrid connectivity.

You need to select the subnet configuration that minimizes wasted IP addresses while meeting all deployment requirements.

Which configuration should you choose?

  1. AzureBastionSubnet: 10.150.0.0/26, GatewaySubnet: 10.150.0.64/27, and Subnet-App: 10.150.0.96/27Answer
  2. B
    AzureBastionSubnet: 10.150.0.0/27, GatewaySubnet: 10.150.0.32/27, and Subnet-App: 10.150.0.64/27
  3. C
    AzureBastionSubnet: 10.150.0.0/26, GatewaySubnet: 10.150.0.64/27, and Subnet-App: 10.150.0.96/28
  4. D
    BastionSubnet: 10.150.0.0/26, GatewaySubnet: 10.150.0.64/27, and Subnet-App: 10.150.0.96/27

Answer

The subnet configuration with AzureBastionSubnet configured as 10.150.0.0/26, GatewaySubnet configured as 10.150.0.64/27, and Subnet-App configured as 10.150.0.96/27.
The correct configuration uses the required name and meets the minimum /26 prefix size for AzureBastionSubnet. For Subnet-App, a /27 prefix provides 32 IP addresses. After subtracting the 5 IP addresses reserved by Azure (the first four and the last one), it provides 27 usable IP addresses, which is sufficient to host 12 virtual machines. GatewaySubnet is configured with a valid /27 prefix.

Step-by-Step Solution

1
Determine the requirements for the Azure Bastion subnet.
The subnet must be named exactly `AzureBastionSubnet` and have a prefix length of at least `/26` (6464 IP addresses). This eliminates configurations using `/27` or incorrect names like `BastionSubnet`.
Azure Bastion enforces strict naming and sizing validation during deployment.
2
Calculate the size requirement for the application subnet.
To host 1212 virtual machines, the subnet must support at least 12+5=1712 + 5 = 17 IP addresses due to Azure reserving the first four and the last IP address of each subnet. A `/28` subnet only contains 1616 total IP addresses (1111 usable), which is insufficient. The smallest valid CIDR block is `/27` (3232 total IP addresses, 2727 usable).
Azure reserves 5 IP addresses per subnet for protocol and routing services.
3
Verify the VPN gateway subnet requirement.
The subnet must be named `GatewaySubnet`. A prefix size of `/27` is supported and valid.
Azure VPN Gateway requires a dedicated subnet named GatewaySubnet, with a recommended size of `/27` or `/28` (minimum `/29`).

Key Concept

Azure Virtual Network subnet sizing constraints, Azure's 5 reserved IP addresses per subnet, and dedicated subnet requirements for Azure Bastion and GatewaySubnet.
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Topics

Create and Configure Virtual Networks and Subnets