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 the two-digit checksum is prefixed. See also Australian Company Number Taxation in Australia List of company registers References External links Australian Business Register ABN Information ABN Lookup Formula of the ABN National identification numbers Taxation in Australia 2000 establishments in Australia