IPv6 for the VMkernel interface is now automatically enabled by default for the latest release of ESXi 5.1 and you may have also noticed the additional IP Address in DCUI after the host boots up.

IPv6 support has been around for awhile now and you can enable IPv6 by using the old vSphere C# Client or the new vSphere Web Client. If you enable or disable IPv6, you will need to perform a system reboot for the changes to go into effect. You also have the ability to enable/disable it via the DCUI, which also has been around for awhile as well.

UPDATE: 07/20/15 - For ESXi 6.0, the VMkernel module is name is now tcpip4 instead of tcpip3.

There is one very important thing to note if you do enable/disable IPv6 on the DCUI, after you made your changes and you wish to apply, there is a very important confirmation box that is displayed.

Carefully read the last sentence which is underline in red "In case IPv6 has been enabled or disabled this will restart your host". If you are not careful in reading the confirmation screen, you may hit yes and your host will issue a reboot. If you are going to use the DCUI to enable or disable IPv6, make sure you do not have any running VMs on your host and you should already have your host maintenance mode when making configuration changes to your host.

In addition to the two methods listed above (vSphere Web Client/C# CLient and DCUI) you can easily enable/disable IPv6 using ESXCLI (my preferred method) and restart the ESXi host when you get a chance.

To view whether IPv6 is currently enabled, run the following ESXCLI command (ESXi 5.5 Update 1 the VMkernel module is now called tcpip4):

esxcli system module parameters list -m tcpip3

As you can see from the screenshot above, ipv6 property is set to 1 which means it is enabled.

To disable IPv6, you just need to set the property to 0, run the following ESXCLI command:

esxcli system module parameters set -m tcpip3 -p ipv6=0

We can now reconfirm by re-running our list operation to ensure the changes were made successfully. All that is left is to perform a system reboot, you can either type in "reboot" or use the new ESXCLI 5.1 command:  

esxcli system shutdown reboot -d 60 -r "making IPv6 config changes"

Note: You can run the ESXCLI command locally on the ESXi Shell or you can run the same command remotely by specifying additional connection options & proxy through vCenter Server if you wish. Take a look here for additional connection options for ESXCLI.