ESXi refuses AD join, "The host does not have a suitable FQDN"

Solution 1:

Read the relevant documentation for your product.

The documentation clearly states that ipv6 is not supported for ESXi -> Active Directory communication. (as of 6.0.)

No ipv6

Solution 2:

Honestly, if this is a standalone host, it's not worth joining it to a domain. Ideally you join your vCenter Server to Active Directory, not the individual hosts.