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Difficulty: MediumPrivate IPv4 Addressing and RFC 1918

A network engineer is configuring internal host interfaces on a private enterprise network. According to RFC 1918, which of the following IP addresses falls within a reserved private IPv4 address block and can be assigned to internal network devices without public Internet routing?

  1. 172.20.45.10Answer
  2. B
    172.32.10.1
  3. C
    192.169.1.25
  4. D
    11.10.0.1

Answer

172.20.45.10 is a valid private IPv4 address as defined by RFC 1918.
The address 172.20.45.10 is part of the RFC 1918 Class B private address range (172.16.0.0/12), which covers all IP addresses from 172.16.0.0 through 172.31.255.255. Since the second octet is 20, it falls within this reserved range and can be used on internal private networks without public routing.

Step-by-Step Solution

1
Recall the three designated RFC 1918 private IPv4 address ranges
The reserved 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).
RFC 1918 specifies exact address ranges reserved strictly for internal private enterprise use.
2
Evaluate the second octet of the Class B candidate address 172.20.45.10
The second octet is 20, which lies inclusively between 16 and 31.
Addresses starting with 172 are only private if the second octet is within the 16 to 31 range inclusive.
3
Verify candidates against public address boundaries
The address 172.20.45.10 is private, while 172.32.10.1, 192.169.1.25, and 11.10.0.1 fall outside the RFC 1918 designated ranges and are public addresses.
Only addresses inside the designated RFC 1918 prefixes are valid private addresses.

Key Concept

RFC 1918 Private IPv4 Address Ranges
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Topics

Private IPv4 Addressing and RFC 1918

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