The numbering ranges for both are complementary, but do not overlap.
Does the BBAN format follow the format specified for this country?
The underlying rules for IBANs is that the account-servicing financial institution should issue an IBAN, as there are a number of areas where different IBANs could be generated from the same account and branch numbers that would satisfy the generic IBAN validation rules. The simple search facility enables you to check bank sort codes, branch addresses, contact details and BIC codes, as well as identifying which banks and institutions accept faster payments.