IPv6 Special Addresses: Reserved, Private (Link-Local / Site-Local), Unspecified and Loopback
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A block of addresses is set aside for private addresses, just as in IPv4, except that like everything in IPv6 the private address block in IPv6 is much larger. These private addresses are local only to a particular link or site and are therefore never routed outside a particular company's network.
Private addresses are indicated by the address having "1111 1110 1 for the first nine bits. Thus, private addresses have a first octet value of FE in hexadecimal, with the next hex digit being from 8 to F. These addresses are further divided into two types based on their scope, described below.
Like IPv4, provision has been made for a special loopback address for testing; datagrams sent to this address loop back to the sending device. However, in IPv6 there is just one address for this function, not a whole block (which was never needed in the first place, really!) The loopback address is 0:0:0:0:0:0:0:1, which is normally expressed using zero compression as ::1.
In IPv4, an IP address of all zeroes has a special meaning; it refers to the host itself, and is used when a device doesn't know its own address. In IPv6 this concept has been formalized, and the all-zeroes address (0:0:0:0:0:0:0:0) is named the unspecified address. It is typically used in the source field of a datagram sent by a device seeking to have its IP address configured. Zero compression can be applied to this address; since it is all zeroes, the address becomes just ::. (I consider this confusing, myself. I think something like 0::0 is a lot more clear, and short enough.)
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