IPv6 Address Assignment Example

In this lesson we’ll take a look how you can create IPv6 prefixes and subnets so that you can configure your entire network with IPv6. We’ll start at the top where IANA (Internet Assigned Numbers Authority) is responsible for the global coordination of the IPv4 and IPv6 address space and move our way all the way to the bottom where we assign subnets and IPv6 addresses to our routers, switches and VLANs.

IPv6 Global Unicast Prefix Assignments

IANA “owns” the entire IPv6 address space and they assign certain prefixes to the RIRs (Regional Internet Registry). There are 5 RIRs at the moment:

rir map

  • AFRINIC: Africa
  • APNIC: Asia/Pacific
  • ARIN: North America
  • LACNIC: Latin America and some Caribbean Islands
  • RIPE NCC: Europe, Middle east and Central Asia

If you are interested, click here for an overview of all IPv6 prefix assignments by IANA.

When a large ISP (or large company) in North America wants IPv6 addresses then they will contact ARIN who will assign them an IPv6 prefix if they meet all requirements. The ISP can then assign prefixes to their customers.

Let’s take a look at some actual prefixes:

IPv6 prefix assignment

  • IANA is using the 2000::/3 prefix for global unicast address space.
  • According to this list, RIPE NCC received prefix 2001:4000::/23 from IANA.
  • A large ISP called Ziggo in The Netherlands receives prefix 2001:41f0::/32 from RIPE NCC.
  • The ISP assigns prefix 2001:41f0:4060::/48 to one of their customers.

Now it’s up to the customer what they want to do with their IPv6 prefix…

IPv6 Global Unicast Subnet Assignments

Our customer received prefix 2001:41f0:4060::/48 and they want to use it to configure IPv6 on their entire network. Where do we start? Take a look at the image below:

IPv6 Global Routing Prefix Subnet Interface ID

The 48-bit prefix that we received is typically called the global routing prefix or site prefix. The interface ID is normally 64 bit which means we have 16 bits left to create subnets.

If I want I can steal some more bits from the Interface ID to create even more subnets but there’s no need for this. Using 16 bits we can create 65.536 subnets …more than enough for most of us. Let’s see what we can do for our customer:

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Forum Replies

  1. Hi Itai,

    It’s probably best to stick to use “A” instead of “10” as it can be confusing. It’s easy to read it as 10 while in reality it’s one, zero.

    About the subnets…the ISP will give you a /48 global prefix that you can use for your network. You should use /64 subnets since it’s convenient for autoconfiguration which leaves you with 16 bits you can use to create different subnets.

    A typical exam question could be something like:

    “The ISP has given you global prefix 2001:41f0:4060::/48. You have five VLANs that require connectivity. What subnets will you use? A

    ... Continue reading in our forum

  2. Hi I had a question. we are always told no more than 500 IP addresses in a subnet yet here we have 18,446,744,073,709,551,616.00 IP addresses. whats the idea on how this should be handled?

    great IPv6 lesson I’m actually enjoying IPv6 for first time in my studies. in past years I actually cringed learning it now im having fun!

  3. Hello Brian

    Yes, I understand your concern. After spending years (and some of us decades) learning and understanding IPv4 with both its strengths and its limitations, it is very often hard to avoid viewing IPv6 in a similar manner.

    Now if you have a prefix length of 48, 64 or even 96 bits which are all very common in IPv6, then of course you will have a subnet capable of supporting an ridiculously enormous number of hosts. Although IPv6 can actually handle a greater number of hosts per subnet than IPv4 (because there are no broadcasts and because it handles a

    ... Continue reading in our forum

  4. Hello Laz,
    Thanks for the reply.

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