20 CFR § 422.112 - Employer identification numbers.
(a) General. Most employers are required by section 6109 of the Internal Revenue Code and by Internal Revenue Service (IRS) regulations at 26 CFR 31.6011(b)-1 to obtain an employer identification number (EIN) and to include it on wage reports filed with SSA. A sole proprietor who does not pay wages to one or more employees or who is not required to file any pension or excise tax return is not subject to this requirement. To apply for an EIN, employers file Form SS-4, “Application for Employer Identification Number,” with the IRS. For the convenience of employers, Form SS-4 is available at all SSA and IRS offices. Household employers, agricultural employers, and domestic corporations which elect social security coverage for employees of foreign subsidiaries who are citizens or residents of the U.S. may be assigned an EIN by IRS without filing an SS-4.
(b) State and local governments. When a State submits a modification to its agreement under section 218 of the Act, which extends coverage to periods prior to 1987, SSA will assign a special identification number to each political subdivision included in that modification. SSA will send the State a Form SSA-214-CD, “Notice of Identifying Number,” to inform the State of the special identification number(s). The special number will be used for reporting the pre-1987 wages to SSA. The special number will also be assigned to an interstate instrumentality if pre-1987 coverage is obtained and SSA will send a Form SSA-214-CD to the interstate instrumentality to notify it of the number assigned.