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IPv6/IPv4 Address Embedding
(Page 2 of 2)
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.
Figure 100: IPv4-Mapped Embedded IPv6 Address Representation
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 Key Concept: IPv4 address embedding is used to create a relationship between an IPv4 address and an IPv6 address to aid in the transition from IPv4 to IPv6. One type, the IPv4-compatible IPv6 address, is used for devices that are compatible with both IPv4 and IPv6; it begins with 96 zero bits. The other, the IPv4-mapped address, is used for mapping IPv4 devices that are not compatible with IPv6 into the IPv6 address space; it begins with 80 zeroes followed by 16 ones.
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Comparing IPv4-Compatible and IPv4-Mapped Embedded IPv6 Addresses
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.
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