IPv6 Address Types

IPv6 looks different than IPv4 but there are some similarities. For example we have unicast addresses and we still have a “public” and “private” range. We use different names for these but the idea is the same. One of the differences is that IPv6 has some additional unicast address types.

We still have multicast, same idea but we use different addresses. There are also some reserved addresses that are similar to their IPv4 counterparts.

Something new is anycast, an address that can be assigned on multiple devices so that packets are always routed to the closest destination. Also, broadcast traffic doesn’t exist in IPv6 anymore.

In this lesson we’ll take a look at all the different address types and I’ll explain what they look like and how we use them.

Unicast

Unicast IPv6 addresses are similar to unicast IPv4 addresses. These are meant to configure on one interface so that you can send and receive IPv6 packets. There are a number of different unicast address types that we’ll discuss here.

Global Unicast

The global unicast IPv6 addresses are similar to IPv4 public addresses. These addresses can be used on the Internet. The big difference with IPv4 is that we have so much address space that we can use global unicast addresses on any device in the network.

Unique Local

Unique local addresses work like the IPv4 private addresses. You can use these addresses on your own network if you don’t intend to connect to the Internet or if you plan to use IPv6 NAT. The advantage of unique local addresses is that you don’t need to register at an authority to get some address space. The FC00::/7 prefix is reserved for unique local addresses, however when you implement this you have to set the L-bit to 1 which means that the first two digits will be FD. Here’s an example:

IPv6 Site Local Address

Let’s discuss all the fields of the unique local address. The first 7 bits indicate that we have a unique local address. 1111 110 in binary is FC in hexadecimal. However, the L bit (8th bit) has to be set to 1 so we end up with 1111 1101 which is FD in hexadecimal.

The global ID (40 bits) is something you can make up. Normally an ISP would choose a prefix but now it’s up to you to think of something. What’s left is 16 bits that we can use for different subnets. This gives us a 64-bit prefix, what’s left is 64 bits for the interface ID.

Let’s work on an example…let’s say that we have a LAN and we want to use unique local IPv6 addresses and we require 10 subnets:

  • The prefix starts with FD.
  • We have 40 bits for the global ID, each hexadecimal character represents 4 bits so we can pick 10 hexadecimal characters. Let’s use AB:1234:5678 as the global ID.
  • Our first subnet will start with 0000.

Here’s what we’ll end up with:

IPv6 site local example

FDAB:1234:5678:0000::/64 will be our first subnet. The other subnets could look like this:

  • FDAB:1234:5678:0000::/64
  • FDAB:1234:5678:0001::/64
  • FDAB:1234:5678:0002::/64
  • FDAB:1234:5678:0003::/64
  • FDAB:1234:5678:0004::/64
  • FDAB:1234:5678:0005::/64
  • And so on…

If you are just messing around with IPv6 then you could use a simple global ID like 00:0000:0000 which is nice because you can shorten it to ::. For production networks, it’s better to pick something that is truly unique. When you want to connect multiple sites that use unique local addresses then you want to make sure you don’t have overlapping global IDs.

Link-Local

Link-local addresses are something new in IPv6. As the wording implies, these addresses only work on the local link, we never route these addresses. These addresses are used to send and receive IPv6 packets on a single subnet.

When you enable IPv6 on an interface then the device will automatically create a link-local address. We use the link-local address for things like neighbor discovery (the replacement for ARP) and as the next hop address for routes in your routing table. You will learn more about this when you work through the static route and OSPFv3 lessons.

We use the FE80::/10 range for link-local addresses, this means that the first 10 bits are 1111 1110 10. Here’s what it looks like:

IPv6 link local structure

The first 10 bits are always 1111 1110 10 which means that we start with FE80. Technically the following are all valid link-local addresses:

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Forum Replies

  1. Hello Brian

    MAC addresses when configured have the U/L bit which is the 7th bit of the 48 bit address. This bit, when set to 0 when this address is locally administered and 1 if the address is globally unique. An example is the virtual MAC address that is created by HSRP. This MAC address will always have 0 in the seventh bit, while a hardwired MAC address on a switch or a PC will have the 7th bit 1.

    Now because there is a mechanism of EUI-64 which is used to assign an IPv6 address that is derived from a MAC addresses, this L bit seems to have migrated into

    ... Continue reading in our forum

  2. Hi Rene,
    I have a silly question running over my head. I see that we have Global unicast address, somewhere i read that the range for global unicast is from 2000::/16 to 3fff::/16.
    My question:

    1. is this correct?
    2. why such a small range of global unicast addresses from a massive IPv6 address? Your introduction to IPv6 course mentioned that there is no real requirement of NAT in IPv6. With this small range we might require NAT in future (please correct me if i am wrong)
    3. can’t we assign other addresses like 7000::/16 as global unicast ?
      I know that IANA does the ad
    ... Continue reading in our forum

  3. Hi Nadav,

    The global IPv6 unicast address space must start with 001 (binary) so that’s correct yes:

    0010 (2000)
    0011 (3000)

    In other words, it has to start with 2000::/3. That covers everything in this range:

    2000:0000:0000:0000:0000:0000:0000:0000-
    3fff:ffff:ffff:ffff:ffff:ffff:ffff:ffff

    That’s 42535295865117307932921825928971026432 addresses in total.

    For each person on the planet we get:

    42535295865117307932921825928971026432 / 7615097670 (world population) = 5.5856534e+27

    So, that’s a crazy number of IPv6 addresses only from the 2000::/3 range :smile:

    Here you can

    ... Continue reading in our forum

  4. Hello Nadav

    Anycast can indeed provide redundancy. With the growth of the Internet, many network services are using Anycast for high-availability requirements, such as DNS and content delivery networks. Anycast has grown in popularity for this purpose.

    ... Continue reading in our forum

  5. well received, thanks for these answers

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