A network administrator is troubleshooting connectivity on an enterprise server host confi… — CompTIA Network+

A network administrator is troubleshooting connectivity on an enterprise server host configured with the IP address $10.205.75.142/21$ . To establish proper firewall routing rules, the administrator needs to identify the complete range of usable host IP addresses within this specific subnet. Which of the following represents the correct usable host IP address range for this subnetwork segment?

$10.205.72.1$ to $10.205.79.254$ Answer
B
$10.205.72.0$ to $10.205.79.255$
C
$10.205.75.1$ to $10.205.75.254$
D
$10.205.64.1$ to $10.205.79.254$

Answer

The usable host range for the

10.205.75.142/21

subnet is

10.205.72.1

10.205.79.254

The CIDR notation

/21

indicates that the first 21 bits represent the network prefix, corresponding to a subnet mask of

255.255.248.0

. In the third octet, subnets increment by multiples of

8

(

256 - 248 = 8

). For an IP address with

75

in the third octet, the network boundary starts at

72

(

9 \times 8 = 72

), making the network address

10.205.72.0

and the broadcast address

10.205.79.255

. Excluding the network and broadcast addresses yields the usable range of

10.205.72.1

through

10.205.79.254

Step-by-Step Solution

Determine the subnet mask and third-octet block size for a /21 CIDR prefix.

A /21 prefix leaves 11 host bits (

32 - 21 = 11

). The subnet mask is

255.255.248.0

. The third octet increment (block size) is calculated as

256 - 248 = 8

The CIDR prefix defines the boundary between the network portion and host portion of the IP address.

Find the network address by calculating the nearest lower multiple of the block size (8) for the third octet (75).

Multiples of 8 in the third octet are

0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80

. Since 75 falls between 72 and 80, the network ID is

10.205.72.0

The network ID is the first IP address of the subnet segment.

Determine the broadcast address for the subnet segment.

The next subnet begins at

10.205.80.0

, making the broadcast address of the current segment

10.205.79.255

The broadcast address is the last address in the subnet block immediately preceding the start of the next block.

Calculate the usable host IP address range by subtracting network and broadcast addresses.

First usable host IP:

10.205.72.1

. Last usable host IP:

10.205.79.254

Usable host addresses consist of all IP addresses between the network address and the broadcast address.

Key Concept

Calculating CIDR Subnet Boundaries and Usable Host Ranges for Non-Byte-Aligned Prefixes

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