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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)                      9  IPv6 Addressing

IPv6 Special Addresses: Reserved, Private (Link-Local / Site-Local), Unspecified and Loopback 1 2 IPv6 Multicast and Anycast Addressing

IPv4-Mapped IPv6 Addresses

These are regular IPv4 addresses that have been mapped into the IPv6 address space, and are used for devices that are only IPv4-capable. They have a set of 16 ones after the initial string of 80 zeroes, and then the IPv4 address. So, if an IPv4 device has the address 222.1.41.90, such as the one in Figure 100, it would be represented as 0:0:0:0:0:FFFF:222.1.41.90, or ::FFFF:222.1.41.90.

The difference between these two kinds of addresses is subtle, but important. The first 80 bits are always zero, so when this is seen you know it is an embedded IPv4 address of some sort. IPv4-compatible IPv6 addresses are only used for devices that are actually IPv6-aware; the IPv4-compatible address is in addition to its conventional IPv6 address. In contrast, if the "FFFF" is seen for the 16 bits after the initial 80, this designates a conventional IPv4 devices whose IPv4 address has been mapped into the IPv6 format. It is not an IPv6-capable device.

IPv6 Special Addresses: Reserved, Private (Link-Local / Site-Local), Unspecified and Loopback 1 2 IPv6 Multicast and Anycast Addressing