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Before June 25, 2011, the Social Security Administration (SSA) had a specific Social Security Number (SSN) numbering scheme. This specific scheme was composed of three parts, Area Number (first three digits), Group Number (middle two digits), and Serial Number (last four digits).
For additional information on the the past SSN numbering scheme:
After June 25, 2011, the SSA switched to the new SSN Randomization scheme when issuing SSNs. In this new scheme, it eliminated the geographical significance of the first three digits of the SSN and eliminated the significance of the highest group number.
Previously unassigned Area Numbers were introduced for assignment except for the Area Numbers 000, 666, and 900-999. Group Numbers will never be 00 and Serial Numbers will never be 0000.
For additional information on the current SSN randomization scheme:
Per the Internal Revenue Service’s (IRS) W-2 instructions, do not accept Individual Taxpayer Identification Numbers (ITINs) in place of a Social Security Number. An ITIN is only available to resident and nonresident aliens who are not eligible for U.S. employment. ITINs are easily identified as they begin with the number 9.
General W-2 Instructions: