Identifying Private IPv4 Addresses Blocked on Internet
Short answer
BD 192.168.18.189 and 172.18.100.78 are within RFC 1918 private address blocks.
Question
Note: You will receive partial credit for each correct selection.
Key Takeaways
The core concept tested is RFC 1918 private address space recognition, with the common trap being confusion between private ranges, multicast addresses, and documentation/test ranges.
This question tests the ability to identify private IPv4 address ranges defined in RFC 1918 that are non-routable on the public internet. It establishes that only addresses within specific Class A, B, and C blocks are considered private.
Common mistake: Learners often incorrectly select option A (203.0.113.x) or C (224.0.x.x), mistaking documentation or multicast ranges for private LAN addresses.
Expert Analysis
Why the Answer Is Correct
According to RFC 1918, three blocks of IPv4 addresses are reserved for private use and must not be routed on the public Internet. These 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)
- 192.168.0.0/16 (192.168.0.0 – 192.168.255.255)
Option B (192.168.18.189) falls within the third range. Option D (172.18.100.78) falls within the second range (since 172.18 is between 172.16 and 172.31). Therefore, B and D are the correct private addresses.
Why the Other Options Are Wrong
Option A (203.0.113.168) belongs to the 203.0.113.0/24 block, which is reserved for documentation and examples (RFC 5737). While it is also non-routable, it is not a "private" address in the RFC 1918 sense used for internal network infrastructure. Option C (224.0.2.172) starts with 224, which identifies it as a Multicast address (Class D), not a unicast private address. Multicast has different routing semantics and is not blocked for the same security/performance reasons as leaked private NAT traffic.
Community Comment Notes
Community consensus correctly identifies RFC 1918 as the governing standard. As Roattana noted, the three designated ranges are strictly defined, and selecting addresses outside these specific subnets leads to errors. The comment highlights the importance of knowing the exact boundaries, especially for the 172.16.0.0/12 block where many candidates mistakenly think only 172.16.x.x is valid.
Official Reference
https://www.rfc-editor.org/rfc/rfc1918.txt
This link opens the official vendor documentation in a new tab.
Exam Strategy
Memorize the three RFC 1918 ranges. For the 172.16.0.0/12 block, remember that any IP starting with 172.16 through 172.31 is private. Do not confuse private addresses with loopback (127.x.x.x) or multicast (224-239.x.x.x) addresses.
Frequently Asked Questions
Why is 203.0.113.168 not considered a private address?
Is all of 172.16.x.x private?
Discussion 1 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 1
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Roattana 👍 1According to RFC 1918, there are three designated ranges for private IPv4 addresses that are non-routable on the public internet: • 10.0.0.0 to 10.255.255.255 (Class A private range) • 172.16.0.0 to 172.31.255.255 (Class B private range) • 192.168.0.0 to 192.168.255.255 (Class C private range)
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