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Difficulty: MediumCreate and Configure Virtual Networks and Subnets

You are designing the IP addressing scheme for a new Azure virtual network named `vnet-retail-prod` that has an address space of 10.10.0.0/2210.10.0.0/22.

You must configure the following subnets:
- A subnet named `AzureBastionSubnet` to deploy an Azure Bastion host.
- A subnet named `snet-app` to deploy exactly 2828 virtual machines.

You need to minimize the address space allocated to each subnet.

Which subnet mask sizes should you assign to `AzureBastionSubnet` and `snet-app`?

  1. `AzureBastionSubnet` with a /26/26 prefix; `snet-app` with a /26/26 prefixAnswer
  2. B
    `AzureBastionSubnet` with a /26/26 prefix; `snet-app` with a /27/27 prefix
  3. C
    `AzureBastionSubnet` with a /27/27 prefix; `snet-app` with a /26/26 prefix
  4. D
    `AzureBastionSubnet` with a /27/27 prefix; `snet-app` with a /27/27 prefix

Answer

Assign a /26/26 prefix to `AzureBastionSubnet` and a /26/26 prefix to `snet-app`.
The correct answer assigns a /26/26 prefix to both `AzureBastionSubnet` and `snet-app`. Azure Bastion requires a dedicated subnet named `AzureBastionSubnet` with a minimum prefix size of /26/26. For the application subnet, since we need to support 2828 VMs, we must account for the 55 reserved IP addresses that Azure allocates in every subnet. This brings the total minimum IP requirement to 28+5=3328 + 5 = 33. A /27/27 subnet provides only 3232 IP addresses, yielding 2727 usable host IPs. Therefore, a /26/26 subnet (which provides 6464 total IPs and 5959 usable host IPs) is the smallest subnet size that can support 2828 virtual machines.

Step-by-Step Solution

1
Determine the minimum subnet size for Azure Bastion.
The subnet must be named exactly `AzureBastionSubnet` and must have a prefix of /26/26 or larger.
Microsoft Azure mandates a minimum prefix size of /26/26 for the Bastion host subnet to ensure adequate IP availability for scaling.
2
Calculate the total IP address requirement for `snet-app` including Azure's reserved IPs.
The total required IP addresses is 28 VMs+5 reserved IPs=3328 \text{ VMs} + 5 \text{ reserved IPs} = 33 IP addresses.
Azure reserves 55 IP addresses in every subnet: the network address (first IP), the default gateway (second IP), two DNS server IPs (third and fourth IPs), and the broadcast address (last IP).
3
Select the smallest subnet mask that accommodates the calculated IP requirement for `snet-app`.
A /27/27 subnet provides 3232 IP addresses (2727 usable), which is not enough. A /26/26 subnet provides 6464 IP addresses (5959 usable), which is sufficient.
To minimize the consumed IP address space, we must select the smallest possible CIDR block that fits 3333 IP addresses, which is /26/26.

Key Concept

Azure subnet IP reservation rules and Azure Bastion subnet sizing requirements.
Estimated Time:1m 30s
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Topics

Create and Configure Virtual Networks and Subnets