When sending ARP request, what must be filled in the target hardware address (THA)? This technet blog post says it is to be filled with zeros (0.0.0.0) -although doesn't look like a MAC address!, and the Wikipedia article suggests FF:FF:FF:FF:FF:FF would be acceptable to all computers, and also stating that This field is ignored in requests.
From the above two articles, I'm given the understanding that there is several implementation for ARP regarding this field. What are the widely used implementation and is there any further standardization for this? And is there any problems so there would be mis-understanding between these implementations? And is there any applications that require specific implementation to that field?