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Difficulty: MediumIPv4 Addressing and Subnetting

A network engineer is provisioning interface IP addresses for a server cluster assigned to the 172.16.140.0/22172.16.140.0/22 IPv4 subnet. Which two IP addresses are valid usable host addresses within this subnet?

  1. A
    172.16.140.0
  2. 172.16.141.255Answer
  3. 172.16.143.254Answer
  4. D
    172.16.143.255

Answer

The addresses 172.16.141.255 and 172.16.143.254 are valid usable host addresses for the 172.16.140.0/22 subnet.
For the network block 172.16.140.0/22, the subnet spans 172.16.140.0 to 172.16.143.255. The unusable boundary addresses are 172.16.140.0 (network ID) and 172.16.143.255 (broadcast ID). Any address strictly between these boundaries (172.16.140.1 to 172.16.143.254) is usable. Consequently, 172.16.141.255 (where host bits are binary 01 11111111) and 172.16.143.254 are both valid usable host addresses.

Step-by-Step Solution

1
Determine the subnet mask and block size from CIDR prefix /22
A /22 prefix corresponds to mask 255.255.252.0. The third octet block size is 256 - 252 = 4.
Prefix length of 22 leaves 10 host bits (32 - 22 = 10). 2 bits in the 3rd octet and 8 bits in the 4th octet belong to host portion.
2
Calculate the network and broadcast boundaries for 172.16.140.0/22
Network Address: 172.16.140.0; Broadcast Address: 172.16.143.255.
140 is a multiple of 4 (4 * 35 = 140). The next subnet begins at 172.16.144.0, so the broadcast address is 172.16.143.255.
3
Determine the usable host IP address range
Usable host range: 172.16.140.1 through 172.16.143.254.
Usable host addresses are all IPs between the network address and broadcast address.
4
Evaluate candidate options against the usable host range
172.16.141.255 and 172.16.143.254 both fall inside the valid range 172.16.140.1 - 172.16.143.254.
172.16.140.0 is the network IP and 172.16.143.255 is the broadcast IP, leaving 172.16.141.255 and 172.16.143.254 as valid usable host IPs.

Key Concept

IPv4 Usable Host Range in Non-Octet Boundary Subnets (/22 Prefix)
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Topics

IPv4 Addressing and Subnetting

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