There is alot of confusion when people see an IPv6 address for the first time. "Why in all that is holy are there alpha characters and what is up with the :’s?" Well the new format is in Hexadecimal not Binary. So we went from base 2 to base 16. From 0’s or 1’s to 0-9 and then A-F! This new 128 bit address space can be rather daunting. I like to think of it in a friendlier way. Now the following is MY interpretation of the addressing and take that with a grain of salt as it may NOT be entirely accurate 🙂
 
    Think of the new address space like a phone number, an international one. v6 addresses are the same. It’s hierarchal in structure. If you want to dial a number in the UK, you dial +044 first. So now in the planet  you know where your calling. Think of the IPv6 address prefix (first 4 positions) to be the equivalent. Just by looking at the first portion of the address you can tell the TYPE of address it is. This is where the similarities fade. You won’t be able to tell WHERE in the world an IPv6 address is, but you can immediately tell what kind it is. We see this same process in IPv4 when looking at things like APIPA addresses (169.254.x.x) and Private non-routable addresses (192.168.x.x, 172.16.x.x, etc..)
 
     Now that this is as clear as mud. Let’s look at some prefixes and get you on the straight and narrow!
 

IPv6 Prefixes

The prefix is the part of the address that indicates the bits that have fixed values or are the bits of the subnet prefix. <– defiinition from Microsoft’s IPv6 document you can download HERE
 

There are three types of IPv6 addresses:

1.   Unicast

A unicast address identifies a single interface within the scope of the type of unicast address. With the appropriate unicast routing topology, packets addressed to a unicast address are delivered to a single interface.

2.   Multicast

A multicast address identifies multiple interfaces. With the appropriate multicast routing topology, packets addressed to a multicast address are delivered to all interfaces that are identified by the address. A multicast address is used for one-to-many communication, with delivery to multiple interfaces.

3.   Anycast

An anycast address identifies multiple interfaces. With the appropriate routing topology, packets addressed to an anycast address are delivered to a single interface, the nearest interface that is identified by the address
 
    Now looking at these types they just happen to be listed in the order of being most common to least common in an infrastructure. We will examine the Unicast addresses here as it’s the most commonly used.
 

  • Global Unicast Addresses

    Global unicast addresses are equivalent to public IPv4 addresses. They are globally routable and reachable on the IPv6 portion of the InternetThe address prefix for currently assigned global addresses is 2000::/3


  • Local-Use Unicast Addresses

    There are two types of local-use unicast addresses:

    1.  Link-local addresses are used between on-link neighbors and for Neighbor Discovery processes. Similar to APIPA addressing in IPv4. Link-local addresses always begin with FE80. With the 64-bit interface identifier, the prefix for link-local addresses is always FE80::/64

    2.  Site-local addresses are used between nodes communicating with other nodes in the same site.Site-local addresses are equivalent to the IPv4 private address space (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16). For example, private intranets that do not have a direct, routed connection to the IPv6 Internet can use site-local addresses without conflicting with global unicast addresses. The first 10-bits are always fixed for site-local addresses (FEC0::/10). These are also re-useable within an organization. Since this can get confusing, Unique-local addresses can be used.

Site-local addresses provide a private addressing alternative to using global addresses for intranet traffic. However, because the site-local address prefix can be used to address multiple sites within an organization, a site-local address prefix address can be duplicated. The ambiguity of site-local addresses in an organization adds complexity and difficulty for applications, routers, and network managers. The first 7 bits have the fixed binary value of 1111110. All unique local addresses have the address prefix FC00::/7
 
Just to round out the remaining types while I type this…
An IPv6 address is easy to classify as multicast because it always begins with “FF”

Anycast IPv6 Addresses

An anycast address is assigned to multiple interfaces. Packets addressed to an anycast address are forwarded by the routing infrastructure to the nearest interface to which the anycast address is assigned. At present, anycast addresses are only used as destination addresses and are only assigned to routers.

 

 

 
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