Archives for October 2013

You probably came to this page because you typed in the command:

• show ipv6 interface
• show ipv6 interface brief

and the output didn’t show you the automatically created EUI-64 Link Local address. Here’s what you are most likely seeing:

By default Cisco doesn’t enable IPv6 on their routers. To enable IPv6 you must do it PER interface! Luckly enabling IPv6 from the command line is pretty easy! The command to enable IPv6 is:

ipv6 enable

You must do this on each of your router interfaces. Here’s a screenshot from the router:

Link Local Addresses are created automatically unless you force a device’s interface to have a specific address. They allow local communication. (ie They are are NOT route-able.)

I am assuming you’ve already checked out my post “Basics of an IPv6 Address”. One of the last things I was explaining was Link Local Addresses. Let’s first look at some Cisco IOS commands.

1. show ipv6 int brief – This will show the IPv6 addresses on the device. We are interested in the Link Local address
2. show int gig0/1 – This will display all kinds of things that relate to the device. We are interested in finding out the MAC address of the device.

I hope you can see that the MAC address and the Link Local address look very similar. When you see this you can automatically assume that it was created by “Extended Unique Identifier 64bit” (EUI-64). They look similar because the Link Local address is partially created from the Mac Address. So how is it automatically made? The rule is:

• Flip the 7th bit. If it is zero change it to a one. If it is a one… flip it to a zero.
• Put FFFE in the middle of the address.

When we look at a Mac address we notice it is a HEX number similar to an IPv6 Address. It is a 48bit address unlike a Interface Identifiers which are 64bit. Essentially it only has 3 groups of numbers and you need 4. To change a MAC address into an Interface Identifier it was agreed upon by the powers that be that we flip the 7th bit and add in a group to make it 64bit.

Using the info above let’s see how the Link Local Address was made. If you need a Decimal to Binary to HEX cheat sheet click the link. It’s half way down the page.

Go back up to the output from the IOS command above. The conversion of the Mac to Interface Identifier checks outs!

For most end user devices the automatically made modified EUI-64 address will be fine. Things like Servers and routers it might be better if you manually specified what their Link Local Address is. There are two ways of doing this:

1. You can change the Mac address to something that will give you something easier to work with like: 1111.1111.1111
2. You can force a Link Local Address.

To force a link local address you need to use the command:

• ipv6 address fe80::1 link-local

Next post will be on two IPv6 protocols: IPv6 Neighbor Discovery Protocol and DAD

First off I want to thank Keith Barker for teaching IPv6. You’ve been instrumental in helping me understand IPv6 and how it’s similar and different from IPv4. This post was written as a journey of me going over the class information.

What is an IPv6 Address?

Keith talks about IPv6 and how it is similar to house addresses. The two main parts of an address are:

1. Street Name – Represents the Network address in IPv6. The network Address is called the “Network Prefix” and tells us about the network groupings of computers.
2. House Address – Represents a single device in IPv6. It is called the “Interface Identifier”.

What does an IPv6 Address look like?

A IPv6 address has several qualities:

1. IPv6 address are 128bits in length.
2. A mask is essentially the dividing line between the network address called the “Network Prefix” and the host address called the “Interface Identifier”.
3. A Mask of /64 following the address will mean that the Network Address is 64bit and the Host Address is 64bits.
4. Most addresses will have a mask of /64
5. Each IPv6 address scan be broken down into 8 groups of 16bits each.
6. Each group is separated by semi-colons.
7. Every 4bits can be converted into a single Hex character. People some times call 4bits a nibble since it’s half of a byte.
8. All leading Zeros can be dropped from the group. More on this near the end of the post.
9. 2 or more groups of consecutive zeros can be merged by putting a :: and then a zero. This can only be done once. More on this near the end of the post.

Before looking at what an IPv6 address normally looks like… it is a good idea to look at what the address looks like in binary:

0010000000000001:0000110110111000:0000000000100001:0000000100010001: 0010000010000001:0000110110101000:0000100000100001:0000000101010001

*Note* – There is no spaces and the address should be all on one line. Due to the size of the address had to fit it on two lines.

How to convert from Binary to Hex.

It’s probably best that you first start off by looking at this Hex Cheat Sheet.

After looking at the Hex cheat sheet you are probably starting to understand why I said in the previous section 4bits can be converted into a Hex character. Using the cheat sheet let’s convert the binary address into the Hexadecimal address that we’ll actually work with on routers and devices.

I’ll break the process up over each 16bits.

Let’s work on the Network Prefix:

0010000000000001:0000110110111000:0000000000100001:0000000100010001

• 0010 0000 0000 0001 – This equals “2001” in hex.
• 0000 1101 1011 1000 – This equals “0DB8” in hex.
• 0000 0000 0010 0001 – This equals “0021” in hex.
• 0000 0001 0001 0001 – This equals “0111” in hex.

We now know the network prefix is: “2001:0DB8:0021:0111”

Let’s work on the Interface Identifier:

0010000010000001:0000110110101000:0000100000100001:0000000101010001

• 0010   0000   1000    0001 – This equals “2081” in hex.
• 0000   1101   1010   1000 – This equals “0DA8” in hex.
• 0000   1000   0010    0001 – This equals “0821” in hex.
• 0000   0001   0101   0001 – This equals “0151” in hex.

This is how the Full IP address will look like in Hex:

2001:0DB8:0021:0111:2081:0DA8:0821:0151/64

Ok. Now that you’ve seen what an IPv6 address looks like… check out the IPv6 cheat sheet:

Let’s take a look at what IPv6 will look like on Cisco IOS. This is the output from setting up an IPv6. Notice the leading zeros had been removed.

You might be wondering 6 things about the line that says: FE80::2D0:BCFF:FE20:7802

1. What is this called? This is a IPv6 Link Local Address
2. How was it entered? – The router “auto-magically” created this address.
3. What is the “::” mean? – This means that several groups of zeros have been shrunk down to shorten the address.
4. Why do some of the groups only have less than 4 characters. – This means that there were leading zeros and were dropped.
5. Is there something special about “FE80”? – Yes. This means this address is a link local address.
6. How did the Interface Identifier get created? – It was created using the MAC address of the device it was assigned to.

I hope I’ve wet your appetite enough to want to learn All About IPv6 Link Local Addresses