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Difficulty: MediumIPv4 Addressing and Subnetting

A network technician is troubleshooting a newly configured workstation that is unable to communicate with its local gateway or external networks. The workstation interface has been manually assigned the IPv4 address 172.31.25.127172.31.25.127 with a prefix length of /26/26. The default gateway interface is configured as 172.31.25.65/26172.31.25.65/26. What is the primary reason for the connectivity failure on this workstation?

  1. The assigned address 172.31.25.127 is the subnet broadcast address for the 172.31.25.64/26 subnetwork and cannot be assigned to an end-device host interface.Answer
  2. B
    The assigned address 172.31.25.127 falls outside the RFC 1918 private IPv4 space and is rejected by the local gateway as a public IP address.
  3. C
    The assigned address 172.31.25.127 represents the network ID for the subnetwork and is reserved strictly for router routing tables.
  4. D
    A /26 prefix length accommodates only 30 usable host addresses per subnetwork, placing 172.31.25.127 into a different subnetwork than 172.31.25.65.

Answer

The assigned address 172.31.25.127 is the subnet broadcast address for the 172.31.25.64/26 subnetwork and cannot be assigned to an end-device host interface.
For a /26 prefix, the subnet block size is 64. The subnetwork containing the gateway 172.31.25.65 starts at network ID 172.31.25.64 and ends at broadcast address 172.31.25.127. Usable host IPs span 172.31.25.65 to 172.31.25.126. Assigning 172.31.25.127 to a host attempts to use the subnetwork broadcast address, which is invalid for host interfaces.

Step-by-Step Solution

1
Determine the subnet mask block size for a /26 prefix.
A /26 prefix corresponds to a subnet mask of 255.255.255.192. The block size in the 4th octet is 256−192=64256 - 192 = 64.
Calculating the block size identifies the subnetwork boundaries.
2
Identify the subnetwork boundaries containing IP address 172.31.25.65.
Subnet multiples of 64 are 0, 64, 128, 192. The IP address 172.31.25.65 falls into the subnetwork starting at 172.31.25.64.
Determines the network ID for the subnetwork.
3
Determine the usable host range and broadcast address for 172.31.25.64/26.
Network ID: 172.31.25.64. Broadcast address: 172.31.25.64+63=172.31.25.127172.31.25.64 + 63 = 172.31.25.127. Usable host range: 172.31.25.65 to 172.31.25.126.
Verifies whether 172.31.25.127 is a valid host address.
4
Evaluate the validity of assigning 172.31.25.127 to a workstation.
Because 172.31.25.127 is the broadcast address, host network stacks will reject or fail to operate correctly with this IP assignment.
Broadcast addresses are reserved for sending packets to all hosts on the subnetwork.

Key Concept

Subnet Boundary and Broadcast Address Identification
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Topics

IPv4 Addressing and Subnetting

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