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Networking Fundamentals · Intermediate

Shared Address Space

The 100.64.0.0/10 block reserved by RFC 6598 for carrier-grade NAT between an ISP and customer routers.

Definition

Shared address space lets ISPs number the link between their CGNAT devices and subscriber equipment without conflicting with customers' RFC 1918 internal networks. It is not RFC 1918 private space and should not be used inside enterprises. A WAN address of 100.64.x.x on a home router indicates CGNAT.

Example

A home router showing WAN IP 100.72.14.9 is behind the ISP's carrier-grade NAT.

Exam tip

100.64.0.0/10 is not private RFC 1918 space, but it is also not publicly routable.

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