Question 349

Neither Public Nor Private

A home router's WAN interface comes up with 100.64.12.9/22 from the ISP. Its LAN is 192.168.1.0/24. Traceroute from inside reaches the internet fine.

A colleague files a bug: "the WAN address is wrong, it is not in any of the three RFC 1918 blocks, so the ISP handed us a public address by mistake."

What is actually happening?

A

The DHCP lease failed and the interface fell back to a self-assigned link-local address

B

100.64.0.0/10 is Shared Address Space from RFC 6598, reserved for carrier-grade NAT and kept deliberately separate from RFC 1918 so it cannot collide with a subscriber's own LAN

C

The RFC 1918 blocks were exhausted, so 100.64.0.0/10 was added to RFC 1918 as a fourth private range

D

It is a globally routable public address; the ISP has assigned one per subscriber