A network administrator is assigning a static IPv4 address to a server interface and needs… — CompTIA Network+

A network administrator is assigning a static IPv4 address to a server interface and needs to configure the appropriate subnet mask in dotted-decimal format. The network documentation specifies that the network segment uses a $/25$ CIDR prefix length. Which of the following dotted-decimal subnet masks corresponds to a $/25$ prefix?

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

Answer

The subnet mask corresponding to a

/25

prefix length is

255.255.255.128

/25

prefix length indicates that 25 bits are allocated to the network portion of the IPv4 address. The first three octets are fully masked (

8 + 8 + 8 = 24

bits, giving

255.255.255

). The 25th bit is the first bit of the fourth octet (

10000000_2

), which evaluates to

128

in decimal. Therefore, the correct subnet mask is

255.255.255.128

Step-by-Step Solution

Determine the distribution of network bits across the four octets for a

/25

prefix length.

The first 24 bits occupy the first three octets (

255.255.255

), leaving 1 network bit for the fourth octet.

An IPv4 address consists of 32 bits divided into four 8-bit octets.

Convert the 8-bit binary representation of the fourth octet to decimal.

Setting the most significant bit gives binary

10000000_2

, which equals

128

in decimal.

The positional value of the highest bit in an 8-bit octet (

2^7

) is 128.

Combine all four octets into dotted-decimal notation.

The completed subnet mask is

255.255.255.128

Putting the binary octets (

11111111.11111111.11111111.10000000

) into standard dotted-decimal formatting yields

255.255.255.128

Key Concept

Conversion of IPv4 CIDR prefix length to dotted-decimal subnet mask

Estimated Time:45s

Question

Answer

Step-by-Step Solution

Key Concept

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