!!! Overview[1]
[{$pagename}] ([ICCID]) are stored in the [SIM]/[UICC] cards and are also engraved or printed on the card body during a process called personalisation. 

[{$pagename}] is defined by the [ITU-T] recommendation [E.118] as the [Primary Account Number].

[{$pagename}] layout is based on [ISO/IEC 7812|ISO 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:
* [Issuer Identification Number] ([IIN]) - Maximum of seven digits:
** [Major Industry Identifier] ([MII]), 2 fixed [digits], 89 for [Telecommunication] purposes.
** [Mobile Network Code] ([MNC]), 1–4 digits, as defined by [ITU-T] recommendation [E.164]
* [Home Location Register] - 2 [digits]
* [Individual Account Identification Number] - length is variable, but every number under one [IIN] will have the same length. This [Account Number] must be a [Unique Identifier] within a [Issuer Identification Number]
* [Check Digit] - Single digit calculated from the other digits using the [Luhn Algorithm].
With the GSM Phase 1 specification using 10 octets into which ICCID is stored as packed BCD, the data field has room for 20 digits with [hexadecimal] digit "F" being used as filler when necessary.

In practice, this means that on [GSM] SIM cards there are 20-digit (19+1) and 19-digit (18+1) [{$pagename}]s 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. 

!! More Information
There might be more information for this subject on one of the following:
[{ReferringPagesPlugin before='*' after='\n' }]
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* [#1] - [Subscriber_identity_module|Wikipedia:Subscriber_identity_module|target='_blank'] - based on information obtained 2016-08-15