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Difficulty: EasyCreate and Configure Virtual Networks and Subnets

An administrator is creating a new virtual network named `VNet1` with an address space of 10.0.0.0/1610.0.0.0/16 to host a new application. The administrator needs to configure subnets for `VNet1`. Which two statements describe correct configurations or requirements for the subnets?

  1. Azure reserves the first four and the last one IP addresses of each subnet's address range.Answer
  2. The address range of any subnet created within `VNet1` must be a subset of the 10.0.0.0/1610.0.0.0/16 address space.Answer
  3. C
    To deploy Azure Bastion, you must create a dedicated subnet named `BastionSubnet` with a prefix of at least /28/28.
  4. D
    To resolve custom domain names for VMs in `VNet1`, a Private DNS Zone is automatically linked to all subnets as soon as the zone is created.

Answer

The correct statements are that Azure reserves the first four and the last one IP addresses of each subnet, and the address range of any subnet must be a subset of the virtual network's address space.
The correct statements are that Azure reserves the first four and the last one IP addresses of each subnet, and the address range of any subnet must be a subset of the virtual network's address space. Azure reserves these five IP addresses within every subnet for network routing, DNS, and DHCP services. Furthermore, any subnet created within a virtual network must fall completely within the address space allocated to the virtual network.

Step-by-Step Solution

1
Review Azure subnet IP reservation rules.
Within any Azure subnet, 5 IP addresses are reserved: the first four (network address, default gateway, two DNS addresses) and the last one (network broadcast address). Thus, the statement regarding Azure reserving the first four and the last one IP addresses is correct.
Azure reserves these for internal protocol and routing purposes.
2
Check the address range constraint for subnets within a virtual network.
Subnets must fall entirely within the address space of their parent virtual network. Therefore, any subnet in VNet1 must use a range within 10.0.0.0/16.
Azure requires subnets to partition or match the VNet address space.
3
Evaluate Azure Bastion requirements.
Azure Bastion requires the subnet name to be exactly 'AzureBastionSubnet' and the size must be /26 or larger. The option stating BastionSubnet and /28 is incorrect.
Azure Bastion has strict naming and size requirements for its gateway subnet.
4
Evaluate Private DNS Zone requirements.
A Private DNS Zone must be manually linked to a virtual network via virtual network links. It is not automatic and applies at the virtual network level, not individual subnets. The option stating that the Private DNS Zone is automatically linked to subnets is incorrect.
Azure DNS private zones require explicit virtual network links to resolve names.

Key Concept

Azure virtual network subnets have strict size constraints, address allocation rules, and reserve 5 IP addresses per subnet.
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Topics

Create and Configure Virtual Networks and Subnets