Examkin

Question

Difficulty: MediumCreate and Configure Virtual Networks and Subnets

You are designing a new Azure virtual network named `vnet-finance-prod` with an address space of 10.200.0.0/2210.200.0.0/22. You need to create a subnet named `snet-app` that must support at least 60 concurrent virtual machine instances. What is the smallest subnet size, represented by the largest CIDR prefix length, that you can assign to `snet-app` to satisfy this requirement?

  1. /25Answer
  2. B
    /26
  3. C
    /27
  4. D
    /24

Answer

The correct subnet size prefix is /25.
The correct prefix is /25 because a /25 subnet provides 128 total IP addresses. Subtracting the 5 IP addresses reserved by Azure yields 123 usable IP addresses, which is the smallest standard prefix that can accommodate the required 60 hosts.

Step-by-Step Solution

1
Determine the required number of IP addresses including Azure's reserved addresses.
Azure reserves 5 IP addresses in every subnet: the first four (network address, default gateway, DNS, and one reserved for future use) and the last one (broadcast address). Therefore, the subnet must support at least 60+5=6560 + 5 = 65 total IP addresses.
To ensure there are enough usable IP addresses for the virtual machines, Azure's management reservations must be added to the requested host count.
2
Find the smallest subnet block (power of 2) that can contain the required total IP addresses.
The powers of 2 are: 3232 (for /27), 6464 (for /26), and 128128 (for /25). Since 65 is greater than 64, the next block size is 128.
IP subnets are allocated in powers of 2, so the block size must be at least 65.
3
Convert the block size to the corresponding CIDR prefix length.
A block size of 128 corresponds to a /25 CIDR prefix (32−log⁡2(128)=2532 - \log_2(128) = 25).
The prefix length determines the mask size, where a larger prefix represents a smaller block size.

Key Concept

Azure virtual networks reserve 5 IP addresses per subnet, which must be factored into subnet sizing calculations.
Estimated Time:1m 30s
Rate this question

Topics

Create and Configure Virtual Networks and Subnets