IPv6 Address Types

Unicast, Multicast, and Anycast

IPv6 does not have broadcasts. What it does have is unicast, multicast, and anycast. Unicast is from one device to one device, one to one. Multicast is from one device to multiple devices, one to many. Anycast is from one device to one of potentially many devices.

As an example of an anycast message: if an anycast message was sent out and it was destined to a particular address, and multiple devices had that same anycast address, it would be the first device, or the closest device, that would respond to that anycast message, and the other devices would not need to do that. So anycast is a one-to-one, but it's a one-to-one out of many different options.

Unicast messages come in several types. One of them is a link local address, one that can communicate on your local area network. There is also a unique local address, and a unique local address communicates on your private network. It can extend beyond the boundaries of just a subnet, but it doesn't extend beyond the boundaries of what is owned equipment. It doesn't get routed on the internet. And then we have the global unicast address, and the global unicast address can go out on the internet because it's globally unique.

Link Local Addresses

If you have a device and it has a network interface on a network that is enabled with IP version 6, then it does get a link local address. Every interface that is capable of doing IP version 6 must have a link local address. Link local addresses are integrated into the core of how IP version 6 works and communicates. Just know for now that IP version 6 requires this link local address.

It is automatically generated, because it's required. It will automatically generate it, and it will be a unique address on the network. There are several ways that it can create this, but it does need to be unique on that local network.

It cannot be routed. So when you have a link local address, it will not go beyond the boundaries of your router. It stays local; it's local communication only. And it's used for things like next hops, so if you're talking about routers and they're communicating, a lot of times they just use link local addresses to communicate back and forth. It also can be used for neighbor discovery. The neighbor discovery protocol is associated with the IP version 6 protocol, and it actually does quite a few things. Neighbor discovery can be used for things like discovering where the default gateway is and how to get out of the network, and it also can be used to discover things like the MAC address of other machines.

How a Device Generates a Link Local Address

So how do your devices generate a link local address? If you have a machine, you plug it into the network, and it has IP version 6 enabled on it, it does need a link local address — it's pretty critical to how IP version 6 operates. There are a couple of different ways to come up with one.

This machine will either randomly generate an IP address for this, and when it's randomly generating, the first 64 bits are specified, so that's going to remain the same, but it's randomly generating the last 64 bits of this address. Or it uses EUI-64, which is a way of taking the MAC address and then making an address out of the MAC address. If it's a MAC address, it's pretty much almost guaranteed to be unique on your network, right? Because the MAC address should be unique to that interface. It's a burned-in address. It's unique in the world. MAC addresses are supposed to be that way — they're not always that way, but they're supposed to be that way. So the idea, if you're using EUI-64, is that you're going to get a unique address with that.

Even with the randomly generated one, you run the risk of accidentally choosing the same IP address that some other device on the network has, and it needs to be unique. So it does go through a process to double check it. We'll talk about those processes of checking this, but just know at this point that there are two ways your device gets a link local address: it either randomly generates it, or it uses a process called EUI-64 to generate an IP address, and it uses the MAC address to generate that.

The Link Local Address Range

What range will be assigned to your computer? When you look at this, you'll see that I've got a table and it's got a bunch of rows to it. That's because we're going to expand upon this and fill this in. But right now I'm just showing you that the link local address is going to start with FE80:, and the first 64 bits are already specified, which means it gives you a range from FE80: — which means all zeros at the end of that — to FE8 colon FF FF FF FFF FF FF FF FFF. That's the last 64 bits all ones in that case right there. So we'll randomly choose a number in between that. That is the link local address range.

The Global Unicast Address

Then you have that global unicast address. The global unicast address is globally unique. So if you have a global unicast address that's on your machine, there is no other device in the world that has that same global unicast address. That allows you to be routed to the outside world and be able to get to other devices.

It seems a little strange that this is an address that's just specifically to your machine, after we've gone years with using private addressing. But at one point in time IP version 4 was the same way: you had to have a specific IP address that was specific to your machine to get out onto the rest of the world. It's just that now a lot of our networks use private addressing. So this global unicast address is not a new concept at all, but we're revitalizing it with IP version 6. For your machine to communicate to the rest of the world using IP version 6, it's going to need a global unicast address.

So although some of that global unicast address is going to be generated from the machine on your network to do the communication, some of it must come from an entity outside, something that is going to assign this to you. Let's say you're in a business and you go to an internet service provider and you ask for a block of addresses that you can use. It could actually vary depending on what your provider is and what your needs are, and there's possibly a cost that's involved in it, so there's going to be a host of different scenarios in here. But generally speaking, what the ISP is going to assign you is the first three hextets. They're going to give you an address that is a /48, the first 48 bits. That's going to be assigned to you.

Then what you will see is that the next 16 bits — so now we're going to a /64 — you have some leeway to assign to subnets, and you can assign these different addresses to subnets. And then the final 64 bits, so from here to the end, with 128 bits total, these then would be assigned to the individual interfaces. So you'll see these /48s and /64s quite often. Most of our networks, when you're talking about IP version 6, are going to be /64s.

So what does that look like from a subnetting standpoint? You're just going to count up. This is what I said would be the subnet hextet right here, and you just count up: 0, 1, 2, 3, 4. And once you get to — once again, it's hexadecimal — 9, a, b, c, d, e, f, and then it's going to count up to 10. So this is going to count up, and subnetting with IP version 6 is much easier than all of the stuff that we have to do for subnetting with IP version 4.

So what does the range look like for a global unicast address? The range is huge. It's going to be 200::/3. That means that the first three bits are going to be 0001, and then it'll be either a zero or a one. So the first bit here is either going to be a two or a three. Those are the two possibilities there. That means our range is going to be anywhere from 200:: to 3 fff and then really all Fs after that. That gives us a possibility of 2 to the 125th possible combinations.

The Unique Local Address

Next type of address we'll talk about is the unique local. When we say local, what we're actually meaning by that is local to your organization. So this could go beyond your subnet — whereas a link local is just your subnet, with a unique local that could be your whole organization. It just cannot be routed to the outside; it cannot be routed globally. It cannot go beyond into your internet service provider or into the rest of the world. And the idea behind it is that these won't get translated. So if you have a machine that's sensitive and you want it to be able to communicate to the rest of your network, but you don't want it to have access to the outside world, this might be a good use of a unique local address.

What does the range of a unique local address look like? It'd be FC0000:. So the first seven bits are going to be FC or FD. So we see the range is FC00: to FD and then all Fs. And what this does is it gives us a range of 2 to the 121st power. That's the number of possible addresses that we have out of this.

Multicast Addresses

Then we have a multicast address — or actually your computer probably has many multicast addresses, and we're going to be getting more in depth into what type of multicast addresses your machine has. But for now, just know that the two that we're most concerned with as we're talking about IPv6 are some well-known multicast addresses that we'll be using, and solicited node multicast addresses, which are a very unique thing to IP version 6. We'll get involved with that solicited node multicast address as well.

So what does the multicast range look like? That is going to be FF00::/8. So the range of it is anywhere from FF00: to all Fs. And that gives us 2 to the 120th power, which means that there are a lot of different combinations with these multicast addresses.

Special Addresses

There are a few special addresses that you should know.

• One of them is the everything, or the default route. The default route looks like ::/0. This is similar to a quad zero that you see with IP version 4. It will be anything from :: to all Fs. This is the whole range, which is why I say it's all, or the default route. So a router will say, if you're going to everything, or anything, then this is the gateway to do that, or this is the next hop to go out to the rest of the world, or this is the default route. So this is the all-inclusive way of saying everything.
• Then we have the unspecified, which is ::/128. The unspecified is a range from :: to ::, so there's only one address. It's all zeros. And what this is saying is "I don't know." Sometimes we'll see it within some of the frames that we send out, or within the packets that we send out, before it assigns certain addresses to it. So this is the "I don't know," or the unspecified.
• Then we have the host itself. The host is ::1/128, so the range is from ::1 to ::1. Just one option here. This specifies the host itself. This is similar to a 127.0.0.1, or the 127.0.0.0 range — the whole range, really. So this specifies the host.

There's the chart as a whole; these are the ones we talked about. The chart itself is actually a lot bigger. There are a lot more addresses, and it specifies specific ranges. We'll talk about a couple more later, but just know that these are the basic ones. This gives you the foundation. In fact, our global unicast addresses have actually opened up, and there are a few more ranges now — pretty much anything that's not defined by something else is a global unicast address. So just know that in general we've got these ranges in there, and this table actually shows you them all in order. It's a good visual to see how they all lay out into this address space.