OCLC Control Number

The OCLC Control Number is anticipated to reach one billion sometime after July
1, 2013. At that point, OCLC will increase the length of the OCLC number to
accommodate a variable length number string.  If you use and/or store OCLC MARC
bibliographic records and the OCLC Control Number, you will notice a change when
the one billion number is reached. You will need to check the systems at your
institution that use OCLC MARC bibliographic records and the OCLC number.  You
may need to implement changes to ensure those systems will be able to
successfully handle the longer OCLC number effective July 1, 2013.

For more specific information on the current and future format of the number,
please see: http://www.oclc.org/us/en/batchprocessing/number-expansion.htm