What’s that % sign after ipconfig IPv6 address?

Reference 1. What’s that % sign after ipconfig IPv6 address?

If that’s you after typing ipconfig on Windows machine with IPv6 enabled then this article is for you. Maybe at first you are looking at this ipconfig output and it seems that there is still no reason for thig %”some number” to be there. I will try to be short in this explanation and explain you crearly why is really there.
That IPv6 address with that strange % sign is called Link-Local IPv6 address. It’s by design an address that has network part same for all computers in the world. That part is called network identifier and is written fe80:0000:0000:0000 or to be short fe80::
So in you have one network card in your comuter everything is working fine and your computer can speak IPv6 to all others in the local network. But!! What if you open your computer and insert second network card NIC to it? Your computer will have two network interfaces and both of them will be with the same network identifier of course fe80:0000:0000:0000. If you go in the netowrking fundamentals you can recall that there can not be a host with two interfaces in the same subnet. Why? If you want to ping the address fe80::5c9f:bc10:bb38:63ec from your computer and your computer has two NICs with addresses fe80::1111:1111:a000:0001 and fe80::5555:5555:5555:1111. Out of what interface will the ping exit the computer? Hm, on both, only on random one? This is not going to work.

To resolve this issue there is the Zone ID. This is our misterious number after the % sign in the IPv6 Link-Local address on your network card. The number is basically an Interface ID so in the network examples below they have Interface in Zone IDs 18, 19, 20, and 21, respectively.

IPv6 Zone ID

This number it distinguishes the network segment by using a numeric zone ID following a percent sign after the IP address:

fe80:3438:7667:5c77:ce27%18

fe80::8939:7684:d84b:a5a4%19

fe80::711b:lc9:f462:7ed4%20

fe80::2112:1234:88a4:b421%21

Zone ID il localy significant. It means that if you want to ping a neighbor computer you will need to specify the neighbor’s IPv6 Link-Local address plus the Zone ID of your computer’s network adapter that is gooing to that computer.