CCIE Study Notes

There's still much to discuss about IPv6 addressing. But first I'd like to make a point about the Aggregate Global Unicast that may not have been obvious in my previous post.

When you're looking at the hex 0x2000, you may wonder why it's special. Until you see it in binary. Yeah, remember when we did subnetting the "long" way by converting to binary? Now think about 0x2001 (which most public IPv6 addresses use.) That would be 0011 in binary. All the 0x2000::/3 addresses would of course include all the 0x2001 address space. Does this make more sense now?

Here's another graphic to chew on:

Now remember the list of "special" addresses. There were many that had FF0X as the first 16 bits. FF for Multicast. 0 for "well - known." And then the X. If 0001 is node local - FF01::1 and FF01::2 make sense that they should be all nodes interface-local and all routers interface-local. FF02::X addresses likewise make sense that they are link-local multicast addresses. So if you break it down and understand how this is put together, it's a lot less memorizing and a lot more understanding.

One special type of multicast address is the Solicited Node address. This address is formed by taking the last (low-order) 24 bits of an address (last 6 characters) - and appending them to the prefix :
FF02:0:0:0:0:1:FF00::/104

Using our example address (from Part 1) >

Looking at it in color - for me - it's easier to remember the 24 bits rather than the 104 bits.

Anycast Address is an address that is assigned to more than one interface (typically belonging to different nodes), and the packet goes to the nearest single instance of that address. These are allocated from the Unicast address space and are "indistinguishable" from other Unicast addresses. If you look at RFC 4291, it says that simply using the same Unicast address on multiple interfaces makes it an Anycast address. It is up to the node to specifically configure it as an Anycast address. So if you get a question that asks which of the following is an Anycast address - simply look for a valid Unicast address. The Cisco page for configuring an Anycast address gives you the step-by-step instructions (for IOS 15.2).

RFC 3513 outlines the Subnet-router anycast address. It has a pre-defined format (but no pre-defined length). Basically, it's the subnet prefix followed by zeros.

One of the relatively short RFCs that you may want to read is RFC 2526 - Reserved IPv6 Subnet Anycast Addresses. It talks about reserving the highest addresses from the subnet as the Anycast address space. Of special note is the decimal 126 - hex 0x7E [Binary 0111 1110] as the Anycast identifiers/reserved addresses for Mobile IPv6 Home-Agents anycast. (Also note the Security section which discusses the security concern of providing possible attackers a well-known address to attack.)

And this is where I am going to end Part 3. Like an infomercial: "But, wait. There's still more!" Yes, there is still much more to cover regarding IPv6 - just to do with addressing. And then there's the difference in headers.