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Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator is assigning IPv4 addresses to host interfaces within a newly provisioned subnet allocated the address block 10.100.16.64/2710.100.16.64/27. Which of the following IPv4 addresses are valid usable host IP addresses within this subnet? (Select TWO.)

  1. 10.100.16.6510.100.16.65Answer
  2. 10.100.16.9410.100.16.94Answer
  3. C
    10.100.16.6410.100.16.64
  4. D
    10.100.16.9510.100.16.95

Answer

The IP addresses 10.100.16.6510.100.16.65 and 10.100.16.9410.100.16.94 are valid usable host addresses within the 10.100.16.64/2710.100.16.64/27 IPv4 subnet block.
For the IPv4 block 10.100.16.64/2710.100.16.64/27, a /27/27 mask yields 3232 total addresses per block. The subnet starts at network ID 10.100.16.6410.100.16.64 and ends at broadcast address 10.100.16.9510.100.16.95. Subtracting these two reserved boundary addresses leaves the usable host address range of 10.100.16.6510.100.16.65 through 10.100.16.9410.100.16.94. Therefore, 10.100.16.6510.100.16.65 and 10.100.16.9410.100.16.94 are both valid usable host IPs.

Step-by-Step Solution

1
Determine the subnet mask and total block size for a /27/27 CIDR prefix.
A /27/27 prefix corresponds to a subnet mask of 255.255.255.224255.255.255.224. The block size per subnet is 256−224=32256 - 224 = 32 total addresses.
Establishing the block size defines the subnet boundaries for network and broadcast calculations.
2
Calculate the network address, broadcast address, and assignable host range for the subnet block.
Network ID: 10.100.16.6410.100.16.64. Broadcast address: 10.100.16.64+31=10.100.16.9510.100.16.64 + 31 = 10.100.16.95. Usable host range: 10.100.16.6510.100.16.65 through 10.100.16.9410.100.16.94.
The network address (first IP) and broadcast address (last IP) are reserved and subtracted from the usable host address pool.
3
Evaluate the given choices against the usable host IP address range.
10.100.16.6510.100.16.65 and 10.100.16.9410.100.16.94 fall within the usable range (10.100.16.6510.100.16.65–10.100.16.9410.100.16.94), while 10.100.16.6410.100.16.64 (Network ID) and 10.100.16.9510.100.16.95 (Broadcast address) are reserved.
Only host addresses falling strictly between the network ID and broadcast address are assignable.

Key Concept

IPv4 Subnet Boundaries and Usable Host Range
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Topics

IPv4 Addressing and Subnetting

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