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Difficulty: HardIPv4 Addressing and Subnetting

A network administrator is provisioning static IP addresses for a cluster of application servers located in the subnet 172.20.100.128/26172.20.100.128/26. Which of the following IPv4 addresses are valid usable host addresses within this designated subnet? (Select TWO.)

  1. A
    172.20.100.128172.20.100.128
  2. 172.20.100.130172.20.100.130Answer
  3. 172.20.100.189172.20.100.189Answer
  4. D
    172.20.100.191172.20.100.191

Answer

The valid usable host addresses are 172.20.100.130172.20.100.130 and 172.20.100.189172.20.100.189.
For the subnet 172.20.100.128/26172.20.100.128/26, the block size is 64 addresses. The network address is 172.20.100.128172.20.100.128 and the broadcast address is 172.20.100.191172.20.100.191. Consequently, the assignable host addresses range from 172.20.100.129172.20.100.129 to 172.20.100.190172.20.100.190. Both 172.20.100.130172.20.100.130 and 172.20.100.189172.20.100.189 lie within this range.

Step-by-Step Solution

1
Determine the block size and network boundaries for the CIDR prefix /26/26.
A /26/26 prefix leaves 6 host bits (32−26=632 - 26 = 6). The total address block size is 26=642^6 = 64.
Calculating block size establishes the start and end boundaries of the IP subnet.
2
Identify the network ID and broadcast address for 172.20.100.128/26172.20.100.128/26.
The network ID is 172.20.100.128172.20.100.128. The broadcast address is 172.20.100.128+64−1=172.20.100.191172.20.100.128 + 64 - 1 = 172.20.100.191.
The first address in the block is reserved for the network ID, and the last address in the block is reserved for the broadcast address.
3
Calculate the usable host IP range.
The first usable IP address is 172.20.100.129172.20.100.129 and the last usable IP address is 172.20.100.190172.20.100.190.
Usable host IP addresses lie strictly between the network ID and the broadcast address.
4
Evaluate the choices against the usable host range (172.20.100.129172.20.100.129 to 172.20.100.190172.20.100.190).
172.20.100.130172.20.100.130 and 172.20.100.189172.20.100.189 are within the usable range, whereas 172.20.100.128172.20.100.128 (network ID) and 172.20.100.191172.20.100.191 (broadcast address) are unusable host addresses.
Assigning network or broadcast addresses directly to hosts results in invalid IP configurations.

Key Concept

IPv4 Subnet Boundaries and Usable Host Range Calculation
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Topics

IPv4 Addressing and Subnetting

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