It is entered in the computers Basic Input Output System (BIOS). It uses a MAC address that is unique to the computer. This is a feature that Dell has called MAC Address Override or MAC Address Pass-Through. Pass-Through Media Access Control (MAC) address, also known as the Virtual MAC address or Auxiliary MAC address. There are many different reasons such as adding it to the approved list, but this article assumes you have a valid reason. Note: This article does not deal with why you need to know this.
