A network engineer is configuring static host IP addresses for infrastructure devices with… — CompTIA Network+

A network engineer is configuring static host IP addresses for infrastructure devices within the newly assigned network block $10.88.16.0/20$ . Which of the following IPv4 addresses represents the last valid, usable host address in this subnet?

$10.88.31.254$ Answer
B
$10.88.31.255$
C
$10.88.32.254$
D
$10.88.30.254$

Answer

10.88.31.254

For the subnet

10.88.16.0/20

, the subnet mask is

255.255.240.0

, giving a block size of 16 in the third octet. The network span is from

10.88.16.0

10.88.31.255

. Reserving

10.88.16.0

for the network ID and

10.88.31.255

for the broadcast address leaves

10.88.16.1

through

10.88.31.254

as usable host addresses. Therefore,

10.88.31.254

is the last valid usable host IP.

Step-by-Step Solution

Calculate the subnet mask and third-octet block size for a /20 prefix.

A /20 prefix corresponds to the mask

255.255.240.0

. The block size in the third octet is

256 - 240 = 16

The third octet contains 4 network bits and 4 host bits (

2^4 = 16

Determine the network boundary and broadcast address.

Starting at network ID

10.88.16.0

, adding 16 to the third octet gives the next subnet starting at

10.88.32.0

. Thus, the broadcast address is

10.88.31.255

The broadcast address is one address before the start of the next subnet.

Identify the last usable host IP address.

Subtracting 1 from the broadcast address (

10.88.31.255 - 1

) yields

10.88.31.254

The last usable host address is always the IP immediately preceding the subnet broadcast address.

Key Concept

IPv4 Subnet Host Boundaries and Allocation

Estimated Time:1m 15s

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