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Difficulty: MediumIPv4 Addressing and Subnetting

A network engineer is configuring static IPv4 addresses for new host interfaces within a server cluster assigned the subnetwork block 172.24.16.0/21172.24.16.0/21. Which TWO of the following IPv4 addresses are valid, usable host addresses within this subnetwork?

  1. 172.24.22.255172.24.22.255Answer
  2. B
    172.24.23.255172.24.23.255
  3. 172.24.16.1172.24.16.1Answer
  4. D
    172.32.20.5172.32.20.5

Answer

The valid usable host addresses for the 172.24.16.0/21172.24.16.0/21 subnetwork are 172.24.22.255172.24.22.255 and 172.24.16.1172.24.16.1.
For a /21 prefix (255.255.248.0255.255.248.0), the third octet increments by 8. The network block starting at 172.24.16.0172.24.16.0 covers 172.24.16.0172.24.16.0 to 172.24.23.255172.24.23.255. The network address is 172.24.16.0172.24.16.0 and the broadcast address is 172.24.23.255172.24.23.255. Therefore, the range of usable host addresses is 172.24.16.1172.24.16.1 through 172.24.23.254172.24.23.254. Both 172.24.16.1172.24.16.1 and 172.24.22.255172.24.22.255 fall within this valid usable host range.

Step-by-Step Solution

1
Determine the subnet mask and block size for CIDR prefix /21.
A /21 prefix corresponds to the dotted-decimal subnet mask 255.255.248.0255.255.248.0. The block size in the third octet is 256−248=8256 - 248 = 8.
Calculating block size establishes the boundary interval for networks incrementing in the third octet.
2
Calculate the network address, broadcast address, and usable host range.
Network ID: 172.24.16.0172.24.16.0; Broadcast address: 172.24.23.255172.24.23.255; Usable host range: 172.24.16.1172.24.16.1 through 172.24.23.254172.24.23.254.
The third octet spans from 1616 up to 16+8−1=2316 + 8 - 1 = 23.
3
Evaluate candidate addresses against the usable host range.
172.24.16.1172.24.16.1 is the first usable host address. 172.24.22.255172.24.22.255 falls strictly between 172.24.16.1172.24.16.1 and 172.24.23.254172.24.23.254. 172.24.23.255172.24.23.255 is the reserved broadcast address. 172.32.20.5172.32.20.5 is outside the network and not in the private RFC 1918 Class B space.
Identify which candidates lie within the inclusive host boundary without matching the network or broadcast ID.

Key Concept

Subnet Boundary and Usable Host Range Calculation for Supernets/Subnets wider than /24
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Topics

IPv4 Addressing and Subnetting

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