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Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator is assigning static IP addresses to server interfaces within the network block 192.168.100.0/25192.168.100.0/25. Which of the following IPv4 addresses are valid, usable host addresses within this subnet? (Select TWO.)

  1. 192.168.100.1192.168.100.1Answer
  2. 192.168.100.126192.168.100.126Answer
  3. C
    192.168.100.0192.168.100.0
  4. D
    192.168.100.127192.168.100.127

Answer

The addresses 192.168.100.1192.168.100.1 and 192.168.100.126192.168.100.126 are the correct usable host IP addresses.
For a CIDR block of 192.168.100.0/25192.168.100.0/25, the address range spans from 192.168.100.0192.168.100.0 through 192.168.100.127192.168.100.127. The network address is 192.168.100.0192.168.100.0 and the broadcast address is 192.168.100.127192.168.100.127. Excluding these two reserved addresses leaves the usable host range as 192.168.100.1192.168.100.1 through 192.168.100.126192.168.100.126. Both 192.168.100.1192.168.100.1 and 192.168.100.126192.168.100.126 fall strictly within this usable range.

Step-by-Step Solution

1
Determine the total block size and network boundary for CIDR prefix /25.
A /25 prefix provides 232−25=27=1282^{32-25} = 2^7 = 128 total IP addresses (192.168.100.0192.168.100.0 to 192.168.100.127192.168.100.127).
The CIDR prefix defines the number of host bits (32−25=732 - 25 = 7 bits).
2
Identify the reserved Network ID and Broadcast address.
The Network ID is 192.168.100.0192.168.100.0 (first address) and the Broadcast address is 192.168.100.127192.168.100.127 (last address).
The first IP address identifies the network itself and the last IP address is reserved for broadcast traffic within the subnet.
3
Calculate the range of usable host IP addresses.
Usable host IP range is from 192.168.100.1192.168.100.1 to 192.168.100.126192.168.100.126, yielding 128−2=126128 - 2 = 126 usable host addresses.
Usable host addresses consist of all IP addresses between the Network ID and the Broadcast address.

Key Concept

Usable IPv4 Host Range Calculation for Subnets
Estimated Time:1m 30s
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Topics

IPv4 Addressing and Subnetting

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