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Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator is assigning static IP addresses to new database servers within an isolated VLAN allocated the block 192.168.50.128/28192.168.50.128/28. Which of the following represent valid, usable host IP addresses that can be assigned to these servers? (Select TWO.)

  1. A
    192.168.50.128192.168.50.128
  2. 192.168.50.130192.168.50.130Answer
  3. 192.168.50.140192.168.50.140Answer
  4. D
    192.168.50.143192.168.50.143

Answer

The addresses 192.168.50.130192.168.50.130 and 192.168.50.140192.168.50.140 are valid usable host IP addresses within the 192.168.50.128/28192.168.50.128/28 subnetwork.
For the network block 192.168.50.128/28192.168.50.128/28, the block size is 1616 (232−282^{32-28}). The network address is 192.168.50.128192.168.50.128 and the broadcast address is 192.168.50.143192.168.50.143. Subtracting these two reserved addresses leaves the usable range of 192.168.50.129192.168.50.129 to 192.168.50.142192.168.50.142. Both 192.168.50.130192.168.50.130 and 192.168.50.140192.168.50.140 lie within this range.

Step-by-Step Solution

1
Determine the block size from the CIDR prefix length
A /28/28 prefix leaves 32−28=432 - 28 = 4 host bits, yielding a block size of 24=162^4 = 16.
Calculating the block size identifies subnet boundary increments.
2
Identify the network ID and broadcast address
The network address is 192.168.50.128192.168.50.128. Adding the block size minus 1 (128+16−1128 + 16 - 1) gives the broadcast address of 192.168.50.143192.168.50.143.
The first and last addresses in a CIDR block are reserved for network identification and subnet broadcasting.
3
Calculate the usable host address range
The usable host range is from 192.168.50.129192.168.50.129 (first usable) to 192.168.50.142192.168.50.142 (last usable).
Usable host addresses exclude the network ID and broadcast address.
4
Evaluate the candidate IP addresses against the usable range
192.168.50.130192.168.50.130 and 192.168.50.140192.168.50.140 fall within [192.168.50.129,192.168.50.142][192.168.50.129, 192.168.50.142]. 192.168.50.128192.168.50.128 is the network ID and 192.168.50.143192.168.50.143 is the broadcast address.
Only IP addresses strictly inside the usable bounds may be assigned to network hosts.

Key Concept

IPv4 Subnet Boundaries and Usable Host Allocation
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Topics

IPv4 Addressing and Subnetting

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