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Difficulty: Very hardPrivate IPv4 Addressing and RFC 1918

A network engineer is configuring interface IP addresses on a core layer switch for an internal corporate segment. Company policy strictly dictates that all internal devices must use RFC 1918 private IPv4 addresses and that addresses must be usable host assignments. Which of the following IP address and subnet mask combinations represents a valid, usable host address located strictly within the upper boundary limit of the RFC 1918 Class B private address space?

  1. 172.31.255.254 255.240.0.0Answer
  2. B
    172.32.0.1 255.240.0.0
  3. C
    172.31.255.255 255.255.0.0
  4. D
    172.15.255.254 255.240.0.0

Answer

The address 172.31.255.254 with subnet mask 255.240.0.0 is the correct choice as it represents the highest usable host address within the RFC 1918 Class B private range.
The correct answer specifies 172.31.255.254 with mask 255.240.0.0 (/12). RFC 1918 reserves 172.16.0.0 to 172.31.255.255 for Class B private networking. In the 172.16.0.0/12 block, 172.31.255.255 is the broadcast address, making 172.31.255.254 the highest usable host address within the private Class B scope.

Step-by-Step Solution

1
Identify the exact RFC 1918 Class B private IPv4 address range boundaries.
RFC 1918 defines the Class B private address space as 172.16.0.0/12172.16.0.0/12, which encompasses IP addresses from 172.16.0.0172.16.0.0 through 172.31.255.255172.31.255.255.
Understanding exact RFC 1918 boundaries is required to filter out public IP addresses.
2
Evaluate candidate addresses against the upper boundary (172.31.255.255172.31.255.255).
172.32.0.1172.32.0.1 exceeds the upper limit and is public. 172.15.255.254172.15.255.254 is below the lower limit (172.16.0.0172.16.0.0) and is public. Both 172.31.255.254172.31.255.254 and 172.31.255.255172.31.255.255 fall inside the private range.
Only addresses between 172.16.0.0172.16.0.0 and 172.31.255.255172.31.255.255 comply with the RFC 1918 private addressing requirement.
3
Determine host address usability for the remaining compliant addresses.
For the network block, 172.31.255.255172.31.255.255 acts as a broadcast address and is unusable for host assignment. 172.31.255.254172.31.255.254 is the last valid, assignable host address in the block.
Host IP assignments must exclude the network ID and broadcast address of the given subnet.

Key Concept

RFC 1918 Private IPv4 Address Space Boundaries and Usable Host Restrictions
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Topics

Private IPv4 Addressing and RFC 1918

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