The IBAN international account number format is based on a standard created by the International Organization for Standardization (ISO). The IBAN account number format contains a unified control mechanism allowing a preliminary check of account numbers before the payment transfer, thus reducing the risk of mistakes in cross-border transfers.
IBAN Structure:
2 country code characters
2 control cipher characters
a maximum of 30 characters for the bank code and account number (different for each country)
(source: http://www.lbbw.cz/en/financial-information-service/useful-info/what-is-iban-what-is-bic.shtml)