The Internal Revenue Service (IRS) assigns tax identification numbers (TINs), also known as employer identification numbers (EINs) or federal tax identification numbers (Federal tax IDs). The main purpose of these numbers is to enable federal authorities to keep track of and identify business entities. Applications for this can be made in various ways, including an online method that is free of charge and offered by the IRS.

These numbers can be assigned to groups or individual entities and are of different types.

TINs can be accepted in the forms of Social Security Numbers (SSNs), Individual Tax Identification Numbers (ITNs) or as EINs and can also be issued by Social Security Administration (SSA) along with the IRS. These numbers would be needed to be filled in any of the tax-related documents for the IRS, tax statements and tax returns for example.
In order to find your own federal tax ID, you can check your W-2 form which should be sent to you by your employer each year. This form would list the tax ID number as social security number (SSN) which can be found in the box–a at the top of the form. In case you cannot get a hold of your social security card, you can always apply for a fresh one with the help of a state ID or driver’s license and the required forms can be filled out at a social security office where you would have to produce a birth certificate and photo ID.
You can also find your employer’s EIN in any of the W-2 forms that have been sent to you by your employer. This number can be found in the box-b at the top of the form. You can also find your employers EIN by making a call to the payroll division.
One question remains is whether one can use a social security number for a tax ID number? Corporations and partnerships are required to have an EIN. But sole proprietors do not require one and instead of the EIN, the IRS permits them to use the social security number.