One of the topics to focus on in IPv6 is the addressing part, greatly because its a totally new addressing scheme. As compared to IPv4 which has 32 bits, the IPv6 address is 128 bits long and is in hexadecimal format (0-9 A-F) or four bits per digit.
0000:0000:0000:0000: 0000:0000:0000:0000
NOTE: In IPv6, due to the length of the address itself there were rules that were made to somehow shorten the address into a bit more human-readable format. These two rules are zero compression and leading zero compression.
zero compression - in case there are consecutive zeros within the address you may replace it by putting in double colons (::). However you can only do this once within an address as the device would have no way of determining how many zeros are there in each '::' theres is if there were more than one. For an example 2001:0005:0000:0000:0201:50FF:FE68:AF50 could be compressed as 2001:0005::0201:50FF:FE68:AF50. The zeros in between were omitted and was replaced by ::.
leading zero compression - for any leading zeros you can go ahead and exclude them to make the address a bit more shorter. However in case there are all zeros within colons you must leave at least one zero to specify that it is all zeros before that hex digit. Using our previous example our address would look like 2001:5:0:0:201:50FF:FE68:AF50.
We could use these two rules simultaneously. And therefore our fully compressed IPv6 address would be 2001:5::201:50FF:FE68:AF50.
One major difference also is there is no ‘class’ system here (class A, B, C, D). In IPv6, we more refer to them as types. Now lets go through them one by one.
Unicast Address – used for sending to one host or interface. Currently there are two types of IPv6 unicast addresses:
Global Unicast – formerly known as Global Aggregatable Unicast address but the ‘Aggregatable’ has now been omitted in the latest RFC. Global Unicast is equal to IPv4s public or Internet address. Knowing this we can understand that this address type will be the ones we use to communicate to the Internet. These addresses composes of the global routing prefix (as of today IANA is assigning numbers that starts with 2000::/3) plus the 64-bit Interface Identifier (EUI-64 format) which we will discuss later.
Link-Local Unicast – are the addresses our devices use to communicate with other nodes on the same local network even without a global unicast address. You may compare this type of address to the layer 2 address or data-link layer address we have in IPv4. Note that these address are autoconfigured on the interfaces using FE80::/10 prefix plus the EUI-64 format Interace Identifier, which again will be discussed later.
Anycast Address – an anycast address is a global unicast address assigned to two or more devices. Packets coming from nodes who wants to access this address will be routed to the closest active device with the anycast address. This is determined by the routing protocol metric or rather the router which receives this packet then routes to the closest one to it.
Multicast Address – a multicast address identifies a group of interfaces. Traffic sent to these addresses are sent to all of the interfaces in that group. Mulicast in IPv6 is not that different in IPv4, its just that in IPv6 only multicast exists. There is no such thing as broadcast in IPv6 (except for some that is specifically addressed to interfaces that maybe within one segment or layer 2 domain like in IPv4). Interfaces may belong to many multicast groups simultaneously. Multicast addresses are addresses that start with FF00::/8. All IPv6 multicast address are within this prefix and so when you see an address that starts with this you will know that this is an IPv6 Multicast address.
Now lets get to the Interface Identifier as promised. Knowing how this address is made is important as you don’t really get to configure this since this is autoconfigured already on the IPv6 enabled interface.
Interface Identifiers (IDs) – are addresses used to identify a unique interface on a link and are sometimes referred to as the ‘host portion’ of the IPv6 address. These address are 64-bits long and is can be dynamically created based on the data-link layer address of the interface. IPv6 Interface IDs are determined depending on the specific data-link layer type of interface there is. In this topic we will discussing Ethernet Interface IDs as this is what we commonly use almost everywhere (even on non-ethernet mediums). Now we can determine its ID based on its MAC address, using a format called Extended Universal Identifier 64-bit (EUI-64). The EUI-64 format Interface ID is derived from the 48-bit MAC address by inserting the hexadecimal digits FFFE between the Organizationally Unique Identifier (OUI), which is the upper three bytes, and the vendor code, which is the lower three bytes of the MAC address. I hope you guys could still remember your MAC addressing fundamentals back in the days because yes it back and used a lot in the IPv6 world. In addition to this the 7th bit in the first byte in the resulting Interface ID, which is the Universal/Local (U/L) bit is always set to binary 1. The U/L bit indicates whether the Interface ID is locally unique on the link or universally (globally) unique. IDs derived from universally unique MAC addresses are assumed to be globally unique so no worries if your already using the Burn In Address of your Interfaces. The 8th bit on the first byte then is the Individual/Group (I/G) bit for managing multicast groups, it is not altered.
As you can see in this example it is pretty easy to understand how an IPv6 Interface ID is composed. You just have to remember two steps. First insert FFFE in between the 48 bit MAC address (in between the two sets of three bytes or 24bits) and then the 7th bit is set to 1. MAC addresses almost always starts with 00 as of yet (I haven’t seen one which isn’t or at least not that I can remember at this time) so you will always see this as ‘02’ (0000 0010).
Now going back to the Glocal Unicast and Link-Local Unicast where we used this Interface IDs. For Global Unicast for example we have a IPv6 public address assigned by APNIC 2001:1F14::/32 (/32s are assigned to ISPs). We now then assign a subnet to our main PoP the subnet 1 and assign a Network Access Server located there an IPv6 address. The server happens to have the MAC address of 00:53:07:2B:AE:09. Knowing this we now determine the Globally unique IPv6 address of this server. The public IPv6 address assigned to us with a subnet of 1 – 2001:1F14::1:, plus the EUI-64 format Interface Identifier derived from the MAC address of the device - 0253:07FF:FE2B:AE09. Our Global Unicast address for our server would be 2001:1F14::1:0253:07FF:FE2B:AE09/64. Now for our Link-Local address we only have to use FE80:: along with our Interface ID. We then now get FE80::0253:07FF:FE2B:AE09. Please note again that this is autogenerated (you will see that once you start assigning an IPv6 address on a router) and that you have to get use to these addresses as these addresses are the ones used by routing protocols for IPv6 e.g OSPFv3, RIPng & MP-BGP.
I hope you guys learned something new and I’ll probably post more topics about IPv6 if I find the time. Yes its been busy these days so it sort of feels good to be able to post a topic again. If you have any questions about this topic just post you may visit the forum.
