Integrated Circuit Card Identifier is defined by the ITU-T recommendation E.118 as the Primary Account Number.
Integrated Circuit Card Identifier layout is based on ISO/IEC 7812. According to E.118, the number is up to 22 digits long, including a single check digit calculated using the Luhn Algorithm.
However, the GSM Phase 1[8] defined the ICCID length as 10 octets (20 digits) with operator-specific structure.
The number is composed of the following subparts:
In practice, this means that on GSM SIM cards there are 20-digit (19+1) and 19-digit (18+1) Integrated Circuit Card Identifiers in use, depending upon the issuer. However, a single issuer always uses the same size for its ICCIDs.
To confuse matters more, SIM factories seem to have varying ways of delivering electronic copies of SIM personalization datasets. Some datasets are without the ICCID checksum digit, others are with the digit.
As required by E.118, The ITU regularly publishes a list of all internationally assigned Issuer Identification Number codes in its Operational Bulletins.