Rules for changing MAC Address

Media Access Control (MAC) is permanent, unique, and assigned by the manufacturer. Spoofing or 'changing' MAC address helps you:

Increase anonymity,
Impersonate other devices and
Bypass filters

(If you do not want to understand the reason behind changing the MAC skip to the last section Summary)

MAC Address is divided into two parts as shown in the image above. It defines:

Organizationally Unique Identifier: First 3-octets defines the organization or the manufacturer of that device. If you search for MAC Address Lookup websites and type the values of the first three octets. You will get information about who manufactured that device.
Network Interface Controller Specific: Last 3-octets tell you about a specific device of that manufacturer.

By combining both parts we get a unique address. For the purpose of spoofing, we need to take care of two things. For the sake of explanation, suppose our MAC Address is E2:22:33:44:55:66.

We just need to focus on the 1st octet i.e. E2. These values are hexadecimal, for better explanation lets convert it into binary which is 11000010. Here we need to understand the last two bits b1 and b0 (as per the diagram above).

b1: Well actually for spoofing the MAC Address we really don't care about this part as far as you want to change it to any random MAC. But here it's 1 which means the device is used for the local administrator. If you change it to 0, means your MAC will be organizationally or globally unique.
b0: This part is most important for spoofing. In general, we need this part to be 0. As in our example, 0 means it will be unicast (sends data or query to only one entity). If you change it to 1 (which is generally not possible for Wi-Fi Adapters) it means it will be multicast (sends data or query to more than one entity).

Summary

[ Possible Values: {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F} (case-insensitive) ]

If you just want to change it to a random and valid MAC address. Type anything from the above possible values. Two values together (form an octet), six octets in total, separated by ':' (colon). Make sure the second value of the first octet is even or {A, C, E}. For example: (where x can be replaced by any values from the possible values set)
- x0:xx:xx:xx:xx:xx
- x2:xx:xx:xx:xx:xx
- x4:xx:xx:xx:xx:xx
- x6:xx:xx:xx:xx:xx
- x8:xx:xx:xx:xx:xx
- xa:xx:xx:xx:xx:xx
- xc:xx:xx:xx:xx:xx
- xe:xx:xx:xx:xx:xx
If you want to change the MAC address so that it looks like from a specific Organization or Company, then search the values for the first three octets from here mac-vendor-list. Example: X:yy:yy:yy

X: Search the company name using Ctrl + F in here mac-vendor-list. (make sure to add a colon after two values)
y: Can be anything from the possible value set mentioned above.