The address length has been increased significantly to expand the available address space. The IPv6 address is 128 bits (or 16 bytes) long, which is four times as long as its predecessor. Because every single bit of added address length doubles the number of addresses available, the size of the IPv6 address space is really huge. It contains 2128 which is about 340 billion billion billion billion different addresses which definitely should suffice for a very long time.

Addresses are written using 32 hexadecimal digits. The digits are arranged into 8 groups of four to improve the readability. Groups are separated by colons. So the written form of IPv6 address looks like this:

2001:0718:1c01:0016:020d:56ff:fe77:52a3

As you can imagine DNS plays an important role in the IPv6 world, because the manual typing of IPv6 addresses is not an easy thing. Some abbreviations are allowed to lighten this task at least a little.Namely: leading zeroes in every group can be omitted. So the example address can be shortened to

2001:718:1c01:16:20d:56ff:fe77:52a3

Secondly, a sequence of all-zero groups can be replaced by pair of colons. Only one such abbreviation may occur in any address, otherwise the address would be ambiguous. This is especially handy for special-purpose addresses or address prefixes containing long sequences of zeroes. For example the loopback address

0:0:0:0:0:0:0:1

may be written as

::1

which is not only much shorter but also more evident. Address prefixes are usually written in the form:

prefix::/length

Where prefix defines the value of bits in the address beginning and length contains the number of important bits from the start. Because the rest of the prefix is not important, zeroes are used in this part of the address, and the “::” abbreviation is deployed. So for example prefix dedicated to the 6to4 transition mechanism is

2002::/16

which means that the starting 16 bits (two bytes, corresponding to one group in the written address) have to contain value 2002 (hexadecimal), the rest is unimportant.

Not all addresses are handled equally. IPv6 supports three different address types for which the delivery process varies:

Unicast (individual) address

identifies one single network interface (typically a computer or similar device). The packet is delivered to this individual interface.

Multicast (group) address

identifies group of interfaces. Data must be delivered to all group members.

Anycast (selective) address

also identifies a group of network interfaces. But this time the packet is delivered just to one single member of the group (to the nearest one).

Broadcast as an address category is missing in IPv6, because broadcast is just a special kind of multicast. Instead of including a separate address category, IPv6 defines some standard multicast addresses corresponding to the commonly used IPv4 broadcast addresses. For example ff02::1 is the multicast address for all nodes connected to given link.

Let’s look at the features of different address types in more detail.