Explanation of the IPv6 address space and other fundamentals.

The main difference of IPv6 over IPv4 is the extended address space, that is roughly 8*10^28 times larger. The reason for address space extension is caused by the IPv4 address exhaustion that became reality in 2011 when IANA allocated the last block of addresses to a regional address authority.

Every endpoint on the Internet requires a unique IP address to realize point-to-point connections. As of 2014, already more than 10*10^9 devices are connected to the Internet. To bypass the fact that there are not enough IPv4 addresses available, technologies like Network Address Translation (NAT) are used. NAT connects full TCP/IP networks using a single registered IPv4 address to the Internet.

IPv4 Address representation and interpretation

An IPv4 address is composed of 4 decimal byte values in the so called dot-decimal notation. There are three IPv4 address types (the examples assume a netmask of 255.255.0.0):

Network address: The least significant bytes (depending on the netmask) are 0. Typically not used in real communication. It is used to describe a network address range.
```
10.41.0.0
```
Broadcast address is used to communicate with all hosts on a network. It can be obtained by performing a bitwise OR operation between the bit complement of the subnet mask and the host's IP address.
```
10.41.255.255
```
Host address: any address between 10.41.0.1 and 10.41.255.254. Host addresses identify a specific host on an IP network and have to be unique.
```
10.41.0.23
```

IPv6 Address representation and interpretation

An IPv6 address consists of 16 bytes organized in blocks of two bytes separated by : The most significant change in representation is the hexadecimal notation. Example:

2002:0a29:0017:0000:0000:ffff:0a29:0017

IPv6 addresses often contain fields of zeros. RFC5952 allows two methods to compress the textual representation to increase readability:

Zero Compression replaces an arbitrary number of consecutive 16-bit groups of zeros with an additional :
```
2002:0a29:0017::ffff:0a29:0017
```
Leading zeros within a 16-bit field can be omitted:
```
2002:a29:17::ffff:a29:17
```

Note: netIP_aton handles the full and compressed text representation.

IPv6 Address Types

In the following section you will find a list of the various address types used in IPv6. The netIF_SetOption function allows you to change the unicast link-local address and a static address (which will typically a unicast global address).

Unicast Global Addresses

IPv6 unicast global addresses are similar to IPv4 public addresses. They are globally routable. The structure of an IPv6 unicast global address creates a three-level topology of public and site information and the local host interface.

3 bits	13 bits	8 bits	24 bits	16 bits	64 bits
001	TLA	Reserved	NLA	SLA	Interface

TLA identifies the highest level in route hierarchy. TLAs are managed by the IANA and registered to Internet registries.
NLA represents the local site or ISP network.
SLA holds the subnet information within a local site.
Interface identifies the specific node on a subnet.

Unicast site-local addresses (FC80::/48)

IPv6 unicast site-local addresses are similar to IPv4 private addresses. The scope of a site-local address is the intercommunication between subnets on an organization's site.

Unicast link-local addresses (FE80::/64)

Addresses in the link-local prefix are only valid and unique on a single link. Within this prefix only one subnet is allocated (54 zero bits), yielding an effective format of fe80::/64. The least significant 64 bits are usually chosen as the interface hardware address constructed in modified EUI-64 format. It can be derived from an Ethernet MAC address.

Unicast loopback address

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

Unicast 6to4 address (2002::/16)

6to4 addresses are used when two IPv6 nodes communicate on an IPv4 network.

00100000 000000010	32 bits	16 bits	64 bits
6to4 prefix	IPv4 Address	SLA	Interface

Neighbor Discovery Protocol and Dynamic Address Assignment

The Neighbor Discovery Protocol (NDP) mainly replaces ARP known from IPv4. The main advantage is that it operates on the ICMP layer and is less dependent on the physical layer.

Rather than using e.g. MAC addresses, NDP uses the link local address. This also allows endpoints to keep router associations even if the global prefix of the network is changed. Link detection is also handled by NDP detecting full or partial link failures or routing failures.

Stateless Address Autoconfiguration (SLAAC) is part of NDP and mostly replaces dynamic address assignment technologies from IPv4 networks like DHCP and Auto-IP. SLAAC assigns the link-local address to an interface and uses this to assign a global IP. Typically this is a unicast address type. Main difference to DHCP is that after a router solicitation (a broadcast on the local network to discover available routers), SLAAC self-assigns an IP address using modified EUI-64. The Duplicate Address Detection tests this new address for duplicates on the network. DHCPv6 can replicate the stateful address assignment trough a single authority on the network to replace or extend SLAAC. It is similar to DHCP on IPv4 networks also advertising additional information like time servers, name servers and bootfile servers which would be missing using SLAAC only.

Note: MDK-Pro supports both, SLAAC and DHCPv6 dynamic address discovery methods.