Question

Difficulty: MediumIPv4 Addressing and Subnetting

A network engineer is allocated the IPv4 address block $172.16.64.0/21$ to provision point-to-point WAN links between branch offices. Each link is configured with a $/30$ subnet mask to conserve address space. What is the maximum number of $/30$ subnets that can be created from this $/21$ block?

Answer: 512 subnets

Answer

The maximum number of

/30

subnets that can be created is 512.

To calculate the total number of subnets created, subtract the starting prefix length from the target prefix length:

30 - 21 = 9

borrowed subnet bits. Applying the formula

2^n

yields

2^9 = 512

subnets.

Step-by-Step Solution

1

Calculate the number of borrowed subnet bits.

9 subnet bits borrowed (

30 - 21 = 9

).

Subnetting a

/21

network block into

/30

subnets requires extending the prefix by 9 bits.

2

Calculate the total number of available subnets.

512 subnets (

2^9 = 512

).

Each borrowed bit doubles the number of created subnets, yielding

2^9 = 512

total subnets.

Key Concept

Calculating the total number of subnets created by borrowing host bits from an IPv4 CIDR prefix.

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Topics

IPv4 Addressing and Subnetting

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