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Difficulty: MediumIPv4 Addressing and Subnetting

A network administrator needs to assign static IP addresses to new servers on a subnet configured with the address block 172.16.40.128/26172.16.40.128/26. Which of the following IP addresses are valid usable host addresses within this subnet? (Select TWO.)

  1. 172.16.40.130Answer
  2. B
    172.16.40.128
  3. 172.16.40.189Answer
  4. D
    172.16.40.191

Answer

The addresses 172.16.40.130 and 172.16.40.189 are valid usable host addresses within the 172.16.40.128/26 subnet.
For the subnet 172.16.40.128/26172.16.40.128/26, the host field contains 32−26=632 - 26 = 6 bits, giving a block size of 26=642^6 = 64 addresses. The network identifier is 172.16.40.128172.16.40.128 and the broadcast address is 172.16.40.191172.16.40.191. Consequently, the assignable host range spans from 172.16.40.129172.16.40.129 to 172.16.40.190172.16.40.190. The values 172.16.40.130172.16.40.130 and 172.16.40.189172.16.40.189 fall squarely inside this valid usable range.

Step-by-Step Solution

1
Determine the subnet block size from the CIDR prefix /26.
A /26 prefix leaves 6 host bits (32−26=632 - 26 = 6). The block size is 26=642^6 = 64 IP addresses.
Calculating the block size identifies the total number of IP addresses contained in the subnet.
2
Identify the network address and broadcast address for the 172.16.40.128 block.
The network address is 172.16.40.128. Adding the block size of 64 gives the next network boundary at 172.16.40.192, so the broadcast address is 172.16.40.191.
The first IP address in the block is reserved as the network identifier and the last IP address is reserved for directed broadcasts.
3
Determine the range of usable host IP addresses.
The usable host range is 172.16.40.129 through 172.16.40.190.
Subtracting 2 reserved addresses (network and broadcast) defines the assignable host IP boundaries.
4
Evaluate the choices against the calculated host range.
172.16.40.130 and 172.16.40.189 are within the range 172.16.40.129–172.16.40.190, whereas 172.16.40.128 is the network ID and 172.16.40.191 is the broadcast address.
Only IP addresses strictly located between the network ID and broadcast address can be assigned to host devices.

Key Concept

Usable IPv4 Host Range Calculation
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Topics

IPv4 Addressing and Subnetting

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