IPv6 link-local address format

Question

I am working on a project related to networking/compression. One of the machines is Windows Vista, which already has IPv6 configured.

When I try ipconfig, I see an address in the following format: fe80::9dc8:72fa:aacd:76e2%10

But when I try to ping this machine from another with ping fe80::9dc8:72fa:aacd:76e2%10, I get the following error:

Ping request could not find host fe80::9dc8:72fa:e327:76e2%10.
Please check the name and try again.

Any ideas/comments are very helpful.

Answer 1

The %10 after the address is called the scope zone. When you use link-local IPv6 addresses, the scope zone is required so that the system knows which interface to send the packet out on.

On Windows, if you issue the netsh interface ipv6 show addresses command, you'll see the addresses assigned to the system complete with their zone IDs. Notice that the zone IDs match the interface index. For example:

Interface 22: VirtualBox Host-Only Network

Addr Type  DAD State   Valid Life Pref. Life Address
---------  ----------- ---------- ---------- ------------------------
Other      Preferred     infinite   infinite fe80::15c3:6bea:aaac:a015%22

This address is scoped %22 because it is on an interface whose index is 22. Similarly, on Linux, you might see a link-local address like fe80::15c3:6bea:aaac:a016%eth0. The format of the zone ID is unique to each individual machine running IPv6, which is why it might be different if you try the ping from the other system.

For example, if you have:

System A (Windows): fe80::15c3:6bea:aaac:a015%22
System B (Linux):   fe80::15c3:6bea:aaac:a016%eth0

... and you want to ping the Linux box from the Windows box, you cannot do ping fe80::15c3:6bea:aaac:a016%eth0. But you can do ping fe80::15c3:6bea:aaac:a016%22. This is the problem. Link-local addresses can be tricky in this way.

Try specifying the correct zone ID. That is, when you do your ping fe80::9dc8:72fa:aacd:76e2%10, first do netsh interface ipv6 show addresses on the machine you are pinging from, and change the %10 to the interface index for whichever interface you want to use on the source system.

If the machine you are pinging from is Linux, you will have to do ping6 -I eth0 fe80::9dc8:72fa:aacd:76e2 (assuming the other system is on eth0), because the Linux command-line utility does not support the % way of specifying the zone (the last time I checked, anyway).

Ideally you should set up an IPv6-capable router on your network to do router advertisements, so that you can use stateless address auto-configuration (SLAAC) and get global unicast addresses. Then this will not be an issue.