A network administrator is assigning subnets for small remote branch offices. Each office … — CompTIA Network+

A network administrator is assigning subnets for small remote branch offices. Each office requires an IPv4 subnet that can support at least $30$ usable host IP addresses. What is the minimum CIDR prefix length (enter the integer prefix number, such as 24 for a $/24$ subnet) that satisfies this requirement?

Answer: 27

Answer

The minimum CIDR prefix length required to support 30 usable hosts is 27.

To host 30 usable devices, a subnet must provide at least 32 total IP addresses (

30

usable

+ 2

reserved for network and broadcast). Using the formula

2^h \ge 32

, we find

h = 5

host bits are needed. Subtracting

5

host bits from the total

32

bits in an IPv4 address yields a prefix length of

27

/27

subnet).

Step-by-Step Solution

Calculate the total number of IP addresses required per subnet including overhead.

30\text{ usable hosts} + 1\text{ network ID} + 1\text{ broadcast ID} = 32\text{ total IP addresses}

Every IPv4 subnet requires two reserved IP addresses: one for the network ID and one for the broadcast address.

Determine the required host bits (

h

2^h = 32 \implies h = 5

host bits.

Five host bits provide

2^5 = 32

total IP addresses.

Calculate the prefix length by subtracting host bits from the total bits in an IPv4 address.

32 - 5 = 27

The prefix length represents the network portion of the address, calculated as

32 - h

Key Concept

Calculating CIDR Subnet Prefix Length for Host Requirements

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