Linux IPv6 HOWTO
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3.1. Addresses without a special prefix

3.1.1. Localhost address

This is a special address for the loopback interface, similiar to IPv4 with its "127.0.0.1". With IPv6, the localhost address is: 

0000:0000:0000:0000:0000:0000:0000:0001

or compressed: 

::1

Packets with this address as source or destination should never leave the sending host.

3.1.2. Unspecified address

This is a special address like "any" or "0.0.0.0" in IPv4 . For IPv6 it's: 

0000:0000:0000:0000:0000:0000:0000:0000

or: 

::

These addresses are mostly used/seen in socket binding (to any IPv6 address) or routing tables.

Note: the unspecified address cannot be used as destination address.

3.1.3. IPv6 address with embedded IPv4 address

There are two addresses which contain an IPv4 address.

3.1.3.1. IPv4-mapped IPv6 address

IPv4-only IPv6-compatible addresses are sometimes used/shown for sockets created by an IPv6-enabled daemon, but only binding to an IPv4 address.

These addresses are defined with a special prefix of length 96 (a.b.c.d is the IPv4 address): 

0:0:0:0:0:ffff:a.b.c.d/96

or in compressed format 

::ffff:a.b.c.d/96

For example, the IPv4 address 1.2.3.4 looks like this: 

::ffff:1.2.3.4

3.1.3.2. IPv4-compatible IPv6 address

Also for sockets, in this case it is for a dual purpose and looks like: 

0:0:0:0:0:0:a.b.c.d/96

or in compressed format 

::a.b.c.d/96

These addresses are also used by automatic tunneling, which is being replaced by 6to4 tunneling.

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