Common
IPv4 shared address space
Special purpose IPv4 network
IPv4 shared address space
An IPv4 shared address space is a block of Internet Protocol version 4 addresses for use by Internet service providers (ISPs) to alleviate the risk of address duplication with downstream private address networks. An example of use is in carrier-grade NAT (CGN) applications. For such special purposes, the Internet Assigned Numbers Authority (IANA) has reserved a /10 size IPv4 address block to be used as shared address space. This block of addresses is specifically meant to be used for implementations of carrier-grade NAT, to connect customer-premises equipment (CPE) to the providers' core routers. Instead of using unique addresses from the rapidly depleting pool of available globally unique IPv4 addresses, ISPs use addresses in 100.64.0.0/10 for this purpose. Because the network between CPEs and the ISP's routers is private to each ISP, all ISPs may share this block of addresses. Devices evaluating whether an IPv4 address is public must be updated to recognize the new address space.
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