IPv6 Part - B-1 : Address Space Forms

I'll start with two useful questions :

Q: Why is the name IPv6 and not IPv5 as successor for IPv4?

A: On any IP header, the first 4 bits are reserved for protocol version. So theoretically a protocol number between 0 and 15 is possible:

• 4: is already used for IPv4

• 5: is reserved for the Stream Protocol (STP, RFC 1819 / Internet Stream Protocol Version 2) (which never really made it to the public)

The next free number was 6. Hence IPv6 was born!

Q: IPv6 addresses: why such a high number of bits?

A:During the design of IPv4, people thought that 32 bits were enough for the world. Looking back into the past, 32 bits were enough until now and will perhaps be enough for another few years. However, 32 bits are not enough to provide each network device with a global address in the future. Think about mobile phones, cars (including electronic devices on its CAN-bus), toasters, refrigerators, light switches, and so on...So designers have chosen 128 bits, 4 times more in length and 2^96 greater in size than in IPv4 today.The usable size is smaller than it may appear however. This is because in the currently defined address schema, 64 bits are used for interface identifiers. The other 64 bits are used for routing. Assuming the current strict levels of aggregation (/48, /32, ...), it is still possible to "run out" of space, but hopefully not in the near future.

- Addresses without a special prefix

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

- 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.

- IPv6 address with embedded IPv4 address

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 ::ffff:a.b.c.d/96
For example, the IPv4 address 1.2.3.4 looks like this: ::ffff:1.2.3.4

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