Format of The ABN
The ABN is an eleven digit number where the first two digits are a checksum. Unlike with the Tax File Number (TFN), the ATO has publicised the formula for checking and creating valid ABN checksums. Also, the nature of the ABN algorithm means that any 9-digit number can be made into a valid ABN.
In the case of companies, the ATO determines the ABN by using the company's Australian Company Number (ACN) to which is prefixed the two-digit checksum.
Read more about this topic: Australian Business Number