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Difficulty: MediumPrivate IPv4 Addressing and RFC 1918

A network administrator is assigning an IPv4 address to an enterprise border router interface connected directly to an Internet Service Provider (ISP) WAN link. The interface must communicate on the public Internet without employing Network Address Translation (NAT). Which of the following IPv4 addresses is globally routable over the Internet according to RFC 1918 specifications?

  1. 172.32.50.1Answer
  2. B
    172.31.200.1
  3. C
    192.168.250.1
  4. D
    10.254.10.1

Answer

172.32.50.1 is a globally routable public address because it lies outside the RFC 1918 private address boundaries.
RFC 1918 defines three private IPv4 address ranges that are not routed on the public Internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. The Class B range covers 172.16.0.0 through 172.31.255.255. The IP address 172.32.50.1 falls outside this range, making it a valid public IPv4 address suitable for Internet communication without NAT.

Step-by-Step Solution

1
Recall the RFC 1918 private IPv4 address allocations.
Class A: 10.0.0.0/8 (10.0.0.0 to 10.255.255.255); Class B: 172.16.0.0/12 (172.16.0.0 to 172.31.255.255); Class C: 192.168.0.0/16 (192.168.0.0 to 192.168.255.255).
Identify which IP ranges are reserved for internal/private use and unroutable on the public Internet.
2
Evaluate each candidate IPv4 address against the RFC 1918 boundaries.
10.254.10.1 is in 10.0.0.0/8 (private). 192.168.250.1 is in 192.168.0.0/16 (private). 172.31.200.1 is in 172.16.0.0/12 (private). 172.32.50.1 is outside 172.16.0.0/12 (public).
The Class B private block ends at 172.31.255.255. Addresses starting at 172.32.0.0 are public IP addresses.
3
Select the address suitable for WAN interface routing without NAT.
172.32.50.1 is public and globally routable.
ISPs drop RFC 1918 private addresses at edge boundaries, so only public IP addresses can communicate over the global Internet without NAT.

Key Concept

RFC 1918 Private Address Ranges and Public Routability
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Topics

Private IPv4 Addressing and RFC 1918

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