A federal taxpayer ID number is what the Internal Revenue Service (IRS) uses to identify businesses. Companies can get a number, officially called an employer identification number (EIN), by completing Form SS-4 or filing an application online.

Requirements

You need an EIN if you have employees, are a corporation or partnership, or withhold taxes on non-wage income paid to non-resident aliens. You also need one if you have a Keogh plan; deal with trusts or real estate mortgage investments; are a nonprofit, farmer's co-op or plan administrator; or file employment, alcohol, tobacco, firearms or excise tax returns.

Sole Proprietors

You need to get a new EIN if a business ownership or structure has changed.

Corporations

You need an EIN if you get a new charter, become a subsidiary, or change to a sole proprietorship or partnership. You also need one if a statutory merger creates a new corporation.

Partnerships

You will need a new EIN if you incorporate, a partnership is changed and run as a sole proprietorship or you end an old partnership and start a new one.

LLC

LLCs need EINs if there are multiple owners or if a single owner is taxed as a corporation or S corporation. An LLC also needs a EIN if it is single-member LLC that was formed with excise-tax-filing requirements for the tax year after Jan. 1, 2008, or with employment-tax requirements for wages paid after January 2009.

Source:

IRS: Do you need a new EIN?

IRS: Employer ID numbers (EIN)