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Difficulty: HardPrivate IPv4 Addressing and RFC 1918

An enterprise network engineer is auditing the IPv4 addressing scheme across several newly acquired branch office routers. Corporate security policy strictly mandates that all internal host interfaces must be configured within RFC 1918 private IPv4 address space to prevent internal routing leakage over the public Internet. During the audit, the engineer inspects four interface IP configurations. Which of the following IPv4 address and subnet mask assignments represents a public IPv4 host address that violates RFC 1918 guidelines?

  1. 172.32.14.50/20Answer
  2. B
    172.31.240.100/20
  3. C
    192.168.254.1/23
  4. D
    10.255.255.254/8

Answer

The address 172.32.14.50/20 is a public IPv4 address because it falls outside the RFC 1918 private Class B prefix range of 172.16.0.0/12.
The RFC 1918 Class B private address space spans from 172.16.0.0 to 172.31.255.255 (CIDR prefix 172.16.0.0/12). The address 172.32.14.50 has a second octet of 32, placing it outside the private address block and into public IPv4 space.

Step-by-Step Solution

1
Recall the defined RFC 1918 private IPv4 address blocks
Class A: 10.0.0.0/8 (10.0.0.0 – 10.255.255.255), Class B: 172.16.0.0/12 (172.16.0.0 – 172.31.255.255), Class C: 192.168.0.0/16 (192.168.0.0 – 192.168.255.255).
Identifying private space requires strict matching against the starting and ending boundaries of RFC 1918 blocks.
2
Evaluate the second octet of the Class B candidate addresses
The address 172.31.240.100 has a second octet of 31, which is inside the range 16–31. The address 172.32.14.50 has a second octet of 32, which is outside the range 16–31.
The /12 prefix for Class B private space covers second octet values from 16 to 31 inclusive (172.16.0.0−172.31.255.255172.16.0.0 - 172.31.255.255).
3
Determine which address violates RFC 1918 private designation
172.32.14.50 is public and cannot be routed across private networks without public IP ownership.
Addresses starting with 172.32.x.x are allocated globally by IANA as public IP addresses.

Key Concept

RFC 1918 Private IPv4 Address Ranges
Estimated Time:1m 30s
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Topics

Private IPv4 Addressing and RFC 1918

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