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Difficulty: MediumIPv4 Addressing and Subnetting

An engineer is allocating static IP addresses for client workstations on a local network segment provisioned with the CIDR block 172.16.50.160/27172.16.50.160/27. Which of the following IP addresses represent valid usable host addresses within this subnet? (Select TWO.)

  1. 172.16.50.165172.16.50.165Answer
  2. B
    172.16.50.160172.16.50.160
  3. 172.16.50.188172.16.50.188Answer
  4. D
    172.16.50.191172.16.50.191

Answer

The IP addresses 172.16.50.165172.16.50.165 and 172.16.50.188172.16.50.188 are valid usable host addresses for the 172.16.50.160/27172.16.50.160/27 subnet.
For the CIDR block 172.16.50.160/27172.16.50.160/27, the prefix length leaves 5 host bits, yielding 25=322^5 = 32 total addresses per subnet. The block starts at network ID 172.16.50.160172.16.50.160 and ends at broadcast address 172.16.50.191172.16.50.191. Subtracting these two reserved addresses gives a usable host IP range of 172.16.50.161172.16.50.161 through 172.16.50.190172.16.50.190. Both 172.16.50.165172.16.50.165 and 172.16.50.188172.16.50.188 fall squarely within this range.

Step-by-Step Solution

1
Calculate the block size and subnet boundaries for a /27/27 prefix length.
A /27/27 mask reserves 32−27=532 - 27 = 5 host bits. The total address block size is 25=322^5 = 32 addresses.
Knowing the block size allows determination of the starting network ID and ending broadcast address.
2
Determine the network address, broadcast address, and usable host IP range.
Network ID = 172.16.50.160172.16.50.160. Broadcast address = 172.16.50.160+31=172.16.50.191172.16.50.160 + 31 = 172.16.50.191. Usable host range = 172.16.50.161172.16.50.161 to 172.16.50.190172.16.50.190.
The network address (all host bits 0) and broadcast address (all host bits 1) are reserved and must be excluded from host assignment.
3
Evaluate the candidate options against the usable host IP range.
172.16.50.165172.16.50.165 and 172.16.50.188172.16.50.188 lie strictly within the range of 172.16.50.161172.16.50.161 to 172.16.50.190172.16.50.190. 172.16.50.160172.16.50.160 is the network ID and 172.16.50.191172.16.50.191 is the broadcast address.
Only IP addresses inside the usable host boundary are assignable to workstation interfaces.

Key Concept

Subnet host range calculation and exclusion of network ID and broadcast address
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Topics

IPv4 Addressing and Subnetting

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