A network administrator is designing an IPv4 addressing plan for a new office department. … — Cisco CCNA

A network administrator is designing an IPv4 addressing plan for a new office department. The department requires static host IP assignments for 61 client workstations, 1 default gateway router interface, and 1 network printer, requiring a total of 63 active host IP addresses. Which subnet mask provides the most efficient address space utilization while satisfying all required host addresses?

$255.255.255.128$ Cevap
B
$255.255.255.192$
C
$255.255.255.224$
D
$255.255.255.0$

Cevap

The subnet mask

255.255.255.128

(corresponding to a

/25

prefix) is the most efficient subnet mask that accommodates 63 host IP addresses.

To support 63 active host interfaces, the subnet must provide at least 63 usable IPv4 addresses. Using 6 host bits yields

2^6 - 2 = 62

usable addresses, which falls 1 short. Therefore, 7 host bits are required (

2^7 - 2 = 126

usable addresses). A subnet with 7 host bits leaves 25 prefix bits (

32 - 7 = 25

), corresponding to the dotted-decimal subnet mask

255.255.255.128

. This is the smallest subnet that satisfies the requirement without wasting excessive IP addresses.

Adım Adım Çözüm

Determine total required host IP addresses

61 \text{ workstations} + 1 \text{ gateway} + 1 \text{ printer} = 63 \text{ usable host IP addresses}

Every device and gateway interface requiring an IP address counts toward the total host capacity.

Calculate required host bits using the usable host capacity formula

2^H - 2 \ge 63

For

H = 6

2^6 - 2 = 62

usable addresses (insufficient). For

H = 7

2^7 - 2 = 126

usable addresses (sufficient).

Two IP addresses in every subnet are reserved for the network ID and broadcast address.

Derive prefix length and dotted-decimal subnet mask

Prefix length

= 32 - 7 = /25

, which converts to

255.255.255.128

Subtracting 7 host bits from 32 total IPv4 bits leaves 25 network bits.

Anahtar Kavram

Usable IPv4 Host Address Calculation (

2^H - 2

) and Subnet Mask Derivation

Soru

Cevap

Adım Adım Çözüm

Anahtar Kavram

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Konular

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