Reasons for introducing IPv6:

• The supply of IPv4 addresses is dwindling. Not many IPv4 addresses remain available for assignment to internet users. According to estimates these will be gone in just a few years.
• The scarcity of addresses in the IPv4 space necessitates the deployment of NAT (Network Address Translator). NAT became necessary with IPv4 since the internet service provider generally assigns only one public IPv4 address, even though multiple devices are operated in the home network. This restriction is no longer required with IPv6. In the IPv6 range the provider assigns not just a single address, but an entire subnet containing at least 2⁶⁴ addresses. This means that every station in the home network can also receive a globally valid IP address. It also simplifies many of the configuration issues raised with NAT. The technique of assigning subnets/prefixes is an option of DHCPv6, called “Prefix Delegation”.
• IPv4 addresses generally have to be configured manually or via DHCP (Dynamic Host Configuration Protocol). IPv6 provides for a simpler and more automated configuration of network devices.

Size of the IPv6 Address Space

IPv6 addresses are 128 bits long. This means that IPv6 makes approximately 340 sextillion (around 3.4 x 10 ³⁸) IPv6 addresses available. By comparison, the IPv4 address space has just over four billion IP addresses.

Structure and Notation of IPv6 Addresses

IPv6 addresses are 128 bits long. They are divided into eight blocks of 16 bits each. They are portrayed in hexadecimal format, with the individual blocks separated from each other by colons.

One or more consecutive blocks with a value of 0 are abbreviated by ::. This abbreviation may be used only once within an IPv6 address. Leading zeros within a block can be left out. Examples:

Notation of IPv6 Networks

When IPv6 networks are rendered, the prefix is given along with its bit length. The prefix is the network address.

Example:

2001:ba98:7654::/48 stands for the IPv6 network with the IP addresses 2001:ba98:7654:0000:0000:0000:0000:0000 to 2001:ba98:7654:ffff:ffff:ffff:ffff:ffff. The prefix is 48 bits long and written 2001:ba98:7654.

Assigning IPv6 Addresses

The internet service provider assigns subnets to its end users. The subnets must be at least /64 in size. That means the prefix may be no longer than 64 bits long. It can be shorter, which makes for a larger subnet. The assignment of prefixes (Prefix Delegation) is part of DHCPv6. For a prefix length of 64 bits, the remaining 64 bits remain available for what is known as the “interface identifier”. The prefix and the interface identifier combine to form the IP address.
The assignment of IPv6 addresses at a network interface is completely automated and independent (Stateless Address Autoconfiguration: SLAAC). Neither a manual IP assignment nor configuration and maintenance of a central service is required for IP assignments. The only requirement is that the terminal device must be connected to the network and started, so that it can automatically integrate itself into the IPv6 network structure.

IPv6 Addresses with Special Properties

When IPv6 is used, each network interface generally receives several IPv6 addresses. Each address has its own properties, which can be identified from the first bits (the “prefix”). A number of important IPv6 address ranges and their properties are described here in simplified form:

• 2001:… designates global IPv6 addresses that are assigned by the internet service provider. IPv6 addresses can only be assigned by the internet service provider if that provider supports IPv6 on your line. The FRITZ!Box and every other connected computer receives an IPv6 address of its own with this prefix for internet communication.
• 2002:… designates global IPv6 addresses when a 6to4 tunnel is used. If your FRITZ!Box uses IPv6 by means of a 6to4 tunnel, the FRITZ!Box and each and every connected computer uses an IPv6 address of its own with this prefix for Internet communication.
• fd00:… designates locally generated IPv6 addresses (Unique Local Addresses – ULA). IPv6 addresses with this prefix cannot be used for internet communication, but only within a local network. If no globally valid prefix is available, for instance because no internet connection is currently active, the FRITZ!Box can assign ULAs in the local network.
• fe80:… designates what “link-local IPv6 addresses (Link Local Addresses)”, which can only be used within a network segment. IPv6 addresses with this prefix are generated by the network interface itself in order to communicate with network interfaces in the immediate vicinity. For the most part these addresses are used to exchange status messages. These addresses are not designed for the exchange of user data.

src: https://service.avm.de/help/en/FRITZ-Box-7390-avme/010/hilfe_ipv6_introduction