Question

Difficulty: MediumIPv4 Addressing and Subnetting

A network engineer is allocated the IPv4 address block $172.30.128.0/20$ for an enterprise campus expansion project. The engineer needs to divide this block into smaller, equal-sized subnetworks using a prefix mask of $/25$ . What is the total number of subnets that can be created from this original $/20$ block?

Answer: 32 subnets

Answer

The total number of subnets that can be created is 32.

Moving from a

/20

prefix to a

/25

prefix borrows 5 host bits (

25 - 20 = 5

). Calculating

2^5

gives 32 distinct subnets.

Step-by-Step Solution

1

Calculate the number of borrowed subnet bits

5 bits borrowed (

25 - 20 = 5

)

The difference between the target prefix length and the starting prefix length represents the network bits borrowed from the host portion.

2

Calculate the total subnet count

32 subnets (

2^5 = 32

)

Each borrowed bit doubles the number of created subnets (

2^n

, where

n

is the number of borrowed bits).

Key Concept

Subnet counting using CIDR prefix borrowing

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Topics

IPv4 Addressing and Subnetting

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