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Difficulty: HardPrivate IPv4 Addressing and RFC 1918

A network engineer is conducting an IP addressing audit for an enterprise network. The organization requires all internal hosts to utilize private IPv4 addresses specified in RFC 1918 to ensure proper isolated routing and avoid packet delivery issues across the public Internet. Which IPv4 address assignment contains a globally routable public IPv4 address rather than a valid RFC 1918 private address?

  1. 172.32.15.1/24 assigned to an internal host interfaceAnswer
  2. B
    172.31.255.254/16 assigned to an internal management interface
  3. C
    192.168.254.1/24 assigned to a virtual LAN gateway interface
  4. D
    10.255.1.1/8 assigned to a core switch loopback interface

Answer

172.32.15.1/24 assigned to an internal host interface is a globally routable public address because it falls outside the RFC 1918 Class B allocation (172.16.0.0 - 172.31.255.255).
The address starting with 172.32.15.1 is a public IPv4 address because the RFC 1918 Class B private address allocation only spans from 172.16.0.0 to 172.31.255.255 (a /12 prefix). Any address with a first octet of 172 and a second octet greater than 31 or less than 16 is a public, globally routable IP address.

Step-by-Step Solution

1
Recall the three designated RFC 1918 private IPv4 address ranges.
The reserved private ranges are: 10.0.0.0/8 (10.0.0.0 – 10.255.255.255), 172.16.0.0/12 (172.16.0.0 – 172.31.255.255), and 192.168.0.0/16 (192.168.0.0 – 192.168.255.255).
Establishing accurate address boundaries is mandatory to distinguish non-routable private IPs from public IPs.
2
Evaluate the second octet of the Class B address candidate 172.32.15.1.
The second octet is 32, which exceeds the upper bound of 31 for the 172.16.0.0/12 private prefix block.
The /12 prefix length covers second octet values from 16 to 31 inclusive (binary 00010000 to 00011111).
3
Verify that all other candidate addresses fall strictly within RFC 1918 boundaries.
172.31.255.254 is within 172.16.0.0/12; 192.168.254.1 is within 192.168.0.0/16; 10.255.1.1 is within 10.0.0.0/8.
Confirming the validity of remaining options isolates the public address misconfiguration.

Key Concept

RFC 1918 Private IPv4 Address Space Boundaries
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Topics

Private IPv4 Addressing and RFC 1918

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