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RFC 6598 / START HERE

Shared CGN address space, before the details

Shared Address Space is the special IPv4 block 100.64.0.0/10. CGN means Carrier-Grade NAT; CPE means Customer Premises Equipment, such as a home router.

A provider can number links between home routers and a provider-operated translator from this shared block. The addresses have a limited scope and need specific packet, routing and DNS boundaries.

Start the guide See the 5 chapters ↓

The idea in one picture

  1. Customer router

  2. Provider shared link

  3. Translate at CGN

A simplified picture of the idea; the chapters explain the actual mechanisms.

Why it exists

It reduces collisions between provider addressing and subscribers’ private networks while supporting IPv4 address sharing. It is distinct from RFC 1918 private space and does not remove the need for IPv6 or solve every NAT limitation.

A familiar example

Imagine several picnic sites reusing the same internal table numbers. Each site’s gate needs to know the local context; an internal number is not a public street address. Shared IPv4 numbers have similar scope limits, and an address label alone does not prove a connection can reach someone.

Helpful background

You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.

Private IPv4 addressesRFC 1918 · beginner overview ↗CIDRRFC 4632 · beginner overview ↗

What you’ll learn next

Begin with chapter one, or choose a topic.

  1. Place shared addresses on the provider linkThe home router’s outside address can belong to a provider’s shared network. It is not necessarily the globally reachable address a remote peer sees.
  2. Recognize the exact /10 blockThe allocation fixes ten high-order IPv4 bits. It spans second octets 64 through 127, with every possible value of the remaining 22 bits.
  3. Handle overlap without borrowing addressesProvider address choices can collide with subscriber networks. Safe use depends on what the equipment can translate and what the provider actually knows.
  4. Contain packets and route advertisementsAddress scope has to be enforced at provider boundaries. Packet filtering and route-advertisement filtering protect different paths.
  5. Keep DNS and reachability in scopeShared addresses should not leak into public DNS or global reverse lookups. Recognizing a special-use address also has limits as an application reachability test.
Start with chapter one →

This introduction explains RFC 6598 (2012). Read the original RFC ↗