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Question

Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator is configuring static IP addresses for user workstations in a newly provisioned branch office segment using the subnet 172.20.48.64/26172.20.48.64/26. Which of the following IPv4 addresses are valid usable host addresses that can be assigned to workstations on this subnet? (Select TWO.)

  1. A
    172.20.48.64172.20.48.64
  2. 172.20.48.75172.20.48.75Answer
  3. 172.20.48.110172.20.48.110Answer
  4. D
    172.20.48.127172.20.48.127
  5. E
    172.20.48.128172.20.48.128

Answer

The valid usable host IP addresses are 172.20.48.75172.20.48.75 and 172.20.48.110172.20.48.110.
For the subnet 172.20.48.64/26172.20.48.64/26, the block size is 6464 IPs (262^6). The network ID is 172.20.48.64172.20.48.64 and the broadcast address is 172.20.48.127172.20.48.127. The valid usable host IP range is from 172.20.48.65172.20.48.65 through 172.20.48.126172.20.48.126. The IP addresses 172.20.48.75172.20.48.75 and 172.20.48.110172.20.48.110 both fall cleanly within this usable host range.

Step-by-Step Solution

1
Calculate the block size and subnet mask from the prefix notation.
A /26/26 prefix leaves 32−26=632 - 26 = 6 host bits. The total address block size is 26=642^6 = 64 addresses, corresponding to a subnet mask of 255.255.255.192255.255.255.192.
Determining the block size establishes the boundary markers for adjacent subnets.
2
Identify the network ID and broadcast address for the subnet.
The network address is 172.20.48.64172.20.48.64. The next subnet begins at 172.20.48.64+64=172.20.48.128172.20.48.64 + 64 = 172.20.48.128. Therefore, the broadcast address for the target subnet is 172.20.48.127172.20.48.127.
The network ID and broadcast address serve specific control functions and are reserved.
3
Determine the usable host IP address range.
The usable range extends from 172.20.48.65172.20.48.65 (first usable host) to 172.20.48.126172.20.48.126 (last usable host).
Subtracting 2 from the total block size (64−2=6264 - 2 = 62) yields the set of valid host addresses.
4
Compare the provided candidate IP addresses against the usable range.
172.20.48.75172.20.48.75 and 172.20.48.110172.20.48.110 lie within the range [172.20.48.65,172.20.48.126][172.20.48.65, 172.20.48.126]. 172.20.48.64172.20.48.64 is the network ID, 172.20.48.127172.20.48.127 is the broadcast address, and 172.20.48.128172.20.48.128 is in the next subnet.
Only IP addresses strictly between the network ID and broadcast address can be statically configured on hosts.

Key Concept

IPv4 CIDR Subnetting and Usable Host Range Calculation

Alternative Method

Calculate total host space as 2^(32-26) = 64 addresses. Identify the subnet range from .64 to .127. Subtract the first (.64) and last (.127) numbers to confirm valid hosts must be between .65 and .126.
Estimated Time:1m 30s
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Topics

IPv4 Addressing and Subnetting

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