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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

Used for automatic tunneling (RFC 2893 / Transition Mechanisms for IPv6 Hosts and Routers), which is being replaced by 6to4 tunneling.

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

or in compressed format

    ::a.b.c.d/96

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