1. Addresses without a special prefix

1.1. Localhost address

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


or compressed:


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

1.2. Unspecified address

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




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.

1.3. IPv6 address with embedded IPv4 address

There are two addresses which contain an IPv4 address.

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):


or in compressed format


For example, the IPv4 address looks like this:


1.3.2. IPv4-compatible IPv6 address

IPv4-compatible IPv6 addresses are used for automatic tunneling (RFC 2893 / Transition Mechanisms for IPv6 Hosts and Routers), which is being replaced by 6to4 tunneling.


or in compressed format