IPv6 Refresher

It might have been a while since you've gone over IPv6, so let's do a little IPv6 refresher. We're going to hit the highlights, at least the highlights that pertain to assigning IPv6 addresses.

The IPv6 address

Here's one of those IP addresses. If you recall, it's 128 bits long, but we use hexadecimal to represent each four bits. So here we have a representation of an IP address using hexadecimal, and it is 32 characters that represent this IP address. The first part of this right here is usually the network address, where the second part is the host bits. There's a couple ways we can assign the host bits, and there's a few ways we can also send the information on the network side.

Ranges reserved for specific purposes

Just like IPv4, IPv6 devotes certain ranges of IPv6 addresses for specific purposes, so let's cover a few of those ranges here when it applies to assigning IP addresses to your different equipment.

First of all, we have a globally unique address. This is very similar to the pre-1996 IP version 4 addresses, in which case when you assigned an IP address to a machine, that was unique to that machine, and there wasn't another, or there wasn't supposed to be at least another machine out there with that same IP address. So it's globally unique in the world, and when you got this globally unique address you can go out and browse the internet, and that's how the traffic knows how to send the traffic back to your specific machine.

We also have the unique local address, and this is something that is intended to just stay in your local area network, or on your local headquarters network. Essentially it's not supposed to go out to the internet; it's supposed to remain on a fairly local basis. It's similar to private addressing, where we have these different ranges and we can just assign it. We don't have to have our internet service provider give us a range to assign to our machines, so we can utilize these internally within our network, similar to IPv4's private addressing. But the difference here is the unique local is never intended to be translated to a globally unique address. It's never meant to go out on the internet; it's only meant to stay local within the network that you control.

Then we have the link local address, and this specifies a specific interface, and then we can do things like ping the interface to see if that interface is up and if the stack is working correctly.

And then we also have this range for multicasting. The range for multicasting is so we can identify different services, to identify specific things that will happen on our network. For instance, the routers have a multicast address, and so when you send out a router advertisement or router solicitation, it's going to be sent to this group, that's going to go out to those specific routers on the network that are part of that group. So that's a multicast group. So we've got these different ranges here that we will see as we get into assigning IP addresses.

Multicast addresses

When it comes to multicast addresses, we also have certain multicast addresses that we'll see commonly on our network. The two that I want to point out for the purpose of this lesson is these first two. The all nodes or hosts on the network, ff02::1. So any device on your network that wants to communicate via IPv6 is going to subscribe to this multicast group, and thereby then they'll receive packets, they'll receive frames for this group on the network.

The idea behind this is that we want to get away from doing actual broadcasts. IPv6 doesn't do broadcast, but they do these multicast addresses, so that way certain machines can subscribe to that network and still get it. So in a sense this becomes a little bit of a broadcast that happens; it's just not going to go across all your links. All your nodes will actually subscribe to this multicasting group.

The next thing is the ff02::2, and this is the all routers. So these two groups are important when it comes to neighbor discovery, and we'll talk more about that a little bit later. But when a router is part of a network, then it will join this ff02::2, and that's important for, once again I say neighbor discovery, but it's also important for SLAAC, because SLAAC uses neighbor discovery. So that is multicasting addresses, and we will once again see this as we get further involved and start looking at the equipment and looking at IP address assignment.

Stateful and stateless assignment

We have a few different options when it comes to getting our machines to have an IPv6 address. First of all, let's cover stateful. Stateful is what we're used to when it comes to DHCP when you're assigning an IPv4 address, and when you say DHCP that's supposed to be really for IPv4. So it's a stateful. What essentially it is is a server or some device is giving a client an IP address. It's stateful: I'm stating that this is what you're going to take. So the stateful is specifying a specific IP address for a client by a server, some sort of server that's on your network.

Versus stateless is a client gets some information regarding the network itself, and then that client comes up with its own IP address, the host bits. So a client is going to get information about the network, and then it's going to develop its own address on that network. So that's stateless.

Stateful is something that is not recommended, but there might be some times that we want to use the stateful addressing, so we will cover it. But mainly stateless is what is really the recommended now with IPv6. Once again, if we were to turn our router or a layer 3 switch into a way to hand out this information, we would probably lean towards using SLAAC. This demo laptop is going to reach out and say, hey, is there a router out there, and a router would then reply and say yes, and this is the network information, and this demo laptop would say, okay, I will create my own host information to get an IPv6 that's acceptable and unique on this network.

There are times when SLAAC doesn't give all of the information that we were going to need for our network. So in that case right there, what we can do is we can use SLAAC with DHCP version 6. Essentially the information that's sent out is, yes, here's the network information for you to create your own address, but also consult with the DHCP version 6 server.

If you want to use just full-on DHCP version 6 and you want to specify specific addresses for your machines, a server will specify and hand out addresses to the machine. It doesn't get away from this demo laptop needing to ask the router some information. Essentially what that does is that there's still some information that needs to come back to that laptop, specifically the router, the default gateway information, so the demo laptop knows how to get that. And some additional information is flagged for that demo laptop, to tell the demo laptop, yes, here is the default gateway, but also go and get your IP address from the DHCP version 6 server.

Neighbor discovery protocol

SLAAC or a stateful DHCP server, both processes rely heavily on this neighbor discovery protocol, and there's a few different frames that can get sent out with the neighbor discovery protocol.

First of all is neighbor solicitation, when a client is asking for information from some other device that's on the network. You've got the neighbor advertisement, which is going to be a response back, and this could be for any machine that's on your network. Next we also have a router solicitation, which is when a client machine asks for information from a router on the network. And then you also have a router advertisement, which is then the response from the router solicitation from one of the devices, one of the routers.

Really, when we're talking about starting out with getting an IP address, we're really talking about this router solicitation and router advertisement. Now, a router advertisement will just go out automatically every so often, every 200 seconds, but what will happen a lot of times is your device won't receive that immediately when you plug it in. What it will do is it will send out a router solicitation, and that router solicitation is asking, on your network, who is a router on this specific network.

Then you will get a response back from the routers, and you could have multiple routers on the network. You have a router advertisement that's sent back with the information about these default gateway right here. That is needed both if you're doing stateless and you're using SLAAC, and it's also used if you're doing stateful: if you have a DHCP version 6 server on your network, it still needs that router information. So that's the starting process of this, if you're using SLAAC to configure this.

Duplicate address detection

Now what happens is this demo laptop gets the router advertisement back with information about the network and about the default gateway on your network, or in our network case it's actually this layer 3 switch. So this device gets information about the network, and now what it's going to do is it's going to automatically create its own address. When it does this, there is a small potential that it actually creates the same address as something else on your network. If that happens, if it creates the same address as something else on your network, that's going to be a problem.

So what we do is we have this duplicate address detection. What it will do is, once this client demo laptop says okay, I think I have an address that's unique, it sends out a neighbor solicitation asking for information from that IP address that it's created. It asks for that out on the network, and then if it doesn't receive a response back, then it assumes that it's going to be a unique address. But if it does receive something back, then it says, oh, that's already been taken, it's already on the network, so I'm not going to configure my device that way.

Now, even though this is really critical from a SLAAC perspective, it still actually does apply if you're using stateful DHCP version 6: an address is given to this demo laptop, and it'll still go through this DAD process to verify that that IP address is not out on the network at that point.

The flags in the router advertisement

When your client machine connects to your network, it's going to send out a router solicitation, and then the router then is going to reply back with a router advertisement, and that advertisement is going to tell that machine what it's going to do next. In that information it gives the default gateway, so no matter what happens now, this demo laptop knows the default gateway. And then that information is going to have some flags in it, and it's going to tell this laptop whether it's going to auto create its IP address for the network, and if it needs to reach out to a DHCP server for additional options.

So what happens is if this router advertisement comes back with SLAAC turned on, essentially this flag right here is the auto configuration. So this is the auto configuration, and what it does is it's telling this demo laptop that you can create your own IP address. So if there's a one in here, then this machine is going to create its own IP address. So it gets that information back, it has the network information, it generates its own host bits on there, and then goes through that SLAAC process to verify that it's unique on this network and no other devices exist out there.

Now, there are times when, once again, we want maybe another server that's sitting off here to the side, and this is a DHCP version 6 server, and it's going to give some additional information. We could also create this DHCP server on one of our Cisco devices too, so there are several options with what is actually the DHCP version 6 server that's handing out the information to this demo laptop. So how this router is going to flag that in the router advertisement is this other flag right here. The other flag, if it's turned on, it's telling this demo laptop to reach out to a DHCP version 6. So that's how that works right there.

Then we also have this managed flag. If we have the managed flag turned on, then what is happening is that this router advertisement is still giving the default gateway information to this demo laptop, but then it's telling this demo laptop, now you need to reach out to your DHCP server for your IP address and any other options that that DHCP version 6 server has on it, and you're not going to create your own IPv6 address in there.

So those are the different flags that go out on the router advertisement to let the client know what it's supposed to do to be able to communicate onto this network.