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Difficulty: HardCreate and Configure Virtual Networks and Subnets

An organization's infrastructure team is planning to deploy a hybrid cloud architecture in Microsoft Azure. The deployment includes a virtual network named VNet-Prod with an address space of 172.16.0.0/16172.16.0.0/16. The virtual network must host three subnets to support the following requirements:

1. A subnet named GatewaySubnet to connect to an on-premises datacenter via an Azure VPN Gateway.
2. A subnet named AzureBastionSubnet to provide secure management access to virtual machines.
3. A subnet named Subnet-App to host application virtual machines. The application tier requires at least 2828 usable IP addresses for virtual machines, plus any Azure reserved addresses.

The team configures the subnets with the following CIDR blocks:
- GatewaySubnet: 172.16.0.0/29172.16.0.0/29
- AzureBastionSubnet: 172.16.1.0/26172.16.1.0/26
- Subnet-App: 172.16.2.0/27172.16.2.0/27

Is the following statement true?
The proposed subnet configuration successfully meets the minimum prefix size requirements for GatewaySubnet and AzureBastionSubnet, and provides enough usable IP addresses for the application tier.

Answer: Answer

Answer

The statement is false because a /27/27 subnet provides only 2727 usable IP addresses in Azure due to the 55 reserved IP addresses, failing to meet the requirement of at least 2828 usable IP addresses for the application tier.
The statement is false because Subnet-App (172.16.2.0/27172.16.2.0/27) provides only 2727 usable IP addresses. In Azure subnets, 55 IP addresses are always reserved (the first four and the last one). A /27/27 subnet has 3232 total IP addresses, so subtracting the 55 reserved addresses leaves only 2727 usable IP addresses, which is fewer than the required 2828 hosts.

Step-by-Step Solution

1
Verify the GatewaySubnet prefix size.
The GatewaySubnet prefix is /29/29.
Azure VPN Gateway requires a GatewaySubnet with a prefix of /29/29 or larger. Therefore, the /29/29 prefix meets the requirement.
2
Verify the AzureBastionSubnet prefix size.
The AzureBastionSubnet prefix is /26/26.
Azure Bastion requires a subnet named AzureBastionSubnet with a prefix of /26/26 or larger. Therefore, the /26/26 prefix meets the requirement.
3
Calculate the number of usable IP addresses for Subnet-App (172.16.2.0/27172.16.2.0/27).
There are 2727 usable IP addresses.
A /27/27 subnet has 232−27=322^{32-27} = 32 total IP addresses. Azure reserves 55 IP addresses per subnet, resulting in 32−5=2732 - 5 = 27 usable IP addresses.
4
Compare the usable IP addresses to the application requirement.
The subnet provides 2727 usable IP addresses, which is less than the required 2828.
Since 27<2827 < 28, the configuration fails to meet the application tier requirements, making the statement false.

Key Concept

Azure virtual network subnet prefix sizing and reserved IP addresses
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Topics

Create and Configure Virtual Networks and Subnets