This part of the IPv6 tutorial discusses the three general IPv6 address types (unicast, multicast, anycast) and introduces the sub type global unicast address.
In my last post in the IPv6 series, you learned the IPv6 address syntax. Today, I will introduce the different types of IPv6 addresses.
IPv6 address types
There are three general types of IPv6 addresses: unicast, multicast, and anycast.
Unicast addresses
You know unicast addresses from IPv4. A unicast address is the most common form of an IP address and is assigned to one network interface.
Multicast addresses
Multicast addresses are also known in IPv4. These addresses identify multiple network interfaces / hosts. A typical use of multicast addresses in a Windows environment is the deployment of OS images to multiple hosts, simultaneously.
Anycast addresses
This is a new address type in IPv6. Like a multicast address, an anycast address identifies multiple interfaces; however, while multicast packets are accepted by multiple machines, anycast packets are delivered only to one interface (host). This address type allows for services that are provided by multiple servers where only one server has to respond. In routing, anycast addresses are used to route packets to the closest routers.
And what about broadcast addresses? They no longer exist in IPv6. Broadcasts are replaced by multicast messages. I will say something about this IPv6 technique in a later post.
IPv6 knows five different unicast address types: global unicast addresses, link-local addresses, site-local addresses, unique local IPv6 unicast addresses, and special addresses.
Global unicast addresses
A global unicast address is simply what we call a public IP address in IPv4—that is, an IP address that is routed across the whole Internet. You can make out a global unicast address easily: The first three bits are set to 001. Thus, the address prefix of a global IPv6 address is 2000::/3 because 0010000000000000 is 2000 in hex. However, in the future, the IANA (Internet Assigned Numbers Authority) might delegate currently unassigned portions of the IPv6 address space. Hence, 2000::/3 won’t always be the prefix for global unicast addresses.
(Note: The diagram is from Microsoft’s “Introduction to IP Version 6.”)
The next 45 bits are the so-called global routing prefix. This is the part that is assigned to organizations. The following 16 bits are for the subnet ID, which you can use for hierarchical addressing in your network. The last 64 bits indicate the interface ID, which is the part of the IPv6 address that must be unique within a subnet. You know what this means, right? You can have 65,536 (=216subnets), and each subnet can have 18446744073709551616 (=264) computers. I hope you have an efficient OS deployment tool. 
In my next post, I will cover the site-local addresses and link-local address.
[IPv6 – Part 1] [IPv6 – Part 2] [IPv6 – Part 3] [IPv6 – Part 4] [IPv6 – Part 5] [IPv6 – Part 6]
