A network engineer is configuring the default gateway interface for a enterprise VLAN assi… — Cisco CCNA

A network engineer is configuring the default gateway interface for a enterprise VLAN assigned the IPv4 network block $172.24.128.0/21$ . According to network policy, the default gateway must be assigned the highest usable IPv4 address within the subnet. Which IPv4 address must be assigned to the gateway interface?

$172.24.135.254$ Answer
B
$172.24.135.255$
C
$172.24.131.254$
D
$172.32.135.254$

Answer

The IPv4 address

172.24.135.254

must be assigned to the gateway interface.

For a

/21

network (

255.255.248.0

), the third octet increments by 8. Given the network ID

172.24.128.0

, the subnet range encompasses

172.24.128.0

through

172.24.135.255

. The broadcast address is

172.24.135.255

, making

172.24.135.254

the highest usable host IPv4 address.

Step-by-Step Solution

Determine the subnet mask and block size for a

/21

prefix

/21

prefix corresponds to a subnet mask of

255.255.248.0

. The block size in the third octet is

256 - 248 = 8

Calculating the block size identifies the boundary for the network increment.

Calculate the network ID and broadcast address

Starting at network ID

172.24.128.0

, adding the block size of

8

gives the next subnet starting at

172.24.136.0

. Therefore, the broadcast address for this subnet is

172.24.135.255

The broadcast address is the last address in the subnet before the start of the next subnet.

Identify the highest usable host IP address

Subtracting 1 from the broadcast address

172.24.135.255

yields

172.24.135.254

as the highest usable host address.

The network ID (

172.24.128.0

) and broadcast address (

172.24.135.255

) are reserved and cannot be assigned to network interfaces.

Key Concept

IPv4 Subnet Boundaries and Usable Host Range Calculation

Question

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Key Concept

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