Are you able to communicate with IPv6? "What is your address?" "Isn't it actually fixed?" Let's check the IPv6 environment at home!

Huh? "Isn't the IPv6 address fixed?"

　Now, I would like you to restart your PC and try accessing "https://www.test-ipv6.com" again.

　The above screen was accessed after rebooting from the same PC as the previous one. You should be able to confirm that the IPv6 address has changed.

Before explaining why the address has changed, some people may be interested, so let me explain a little about the difference in the number of digits in the address after rebooting.

　In IPv6, there is a rule that consecutive 0's can be omitted in the address. Notation such as "::" often appears, but this is a pattern in which 0000 of ":0000:" is omitted. The above address after restarting has a different number of digits due to the "5b3" part. The original value is "05b3".

So, why have IPv6 addresses changed? "Let's get back to talking about it."

　Here, let's check the IPv6 address from the Windows 10 settings screen. If you open [Ethernet (for wired)] from [Network and Internet] in [Settings] and select a connection destination such as [Network 2], you can check the IPv6 address in the property item.

　The following screen is the same PC properties as above.

Yes. Another value appeared.

　What is right? It's a little confusing, but both of these are valid IPv6 addresses, and both are actually values ​​that can be used for communication.

As a test, if you try PING from another PC in the same network*, you can confirm that you can communicate with either address.

*Author's note: It is necessary to enable the reception rule for "File and printer sharing (echo request/ICMPv6 reception)" on the firewall on the receiving side in advance.

　To understand this situation, open the properties of [Ethernet Connection] from [Network and Sharing Center]. It's quick and easy to check or use a command.

By starting Windows PowerShell or Command Prompt and executing "ipconfig.exe", you can check the IP address assigned to your PC. If you read here, you can see that the previous address is as follows.

　Those who have glimpsed the explanation of IPv6 are concerned with "EUI-64", which is that the IPv6 address consists of a combination of the prefix descended from the upper level and the interface ID generated from the MAC address. You may have heard the story.

　Speaking of IPv6 addresses, aren't they basically global and fixed addresses? "Some people may have this question." However, in fact, in Windows 10, a mechanism called "temporary IPv6 address" is used as the IPv6 address for normal communication. The address is about to change.

　This is to ensure anonymity on the Internet. Since IPv6 addresses are globally unique addresses, using EUI-64 addresses generated from the MAC address of the interface makes it easy to identify the source.

　Using a changing temporary IPv6 address for normal communication makes such identification difficult. Therefore, the temporary IPv6 address was displayed at "test-ipv6.com" when connecting to the Internet.

　There is also another link-local IPv6 address (value starting with fe80::; %14 at the end is not required) on the screen above. This is an address that is valid only on the same link. You can also use this address to communicate within your local network.

　By the way, although it may not be used often, the temporary IPv6 address can be disabled with the following command (executed from PowerShell with administrator privileges).

Set-NetIPv6Protocol -UseTemporaryAddresses Disabled