This is called a "Sole Proprietor".

The EIN number is required in all fifty states and allows you to change from a sole proprietor where if you do not; you can get legally sued should a deal go bad.

I have even heard of a complete entity being reclassified by the IRS as a dealer and therefore having to pay different taxes at higher levels.I bring this up here because it's conceivable that if a person held both flipping properties and long term rentals in a sole proprietor or in their personal name, because they were co-mingling property types, the IRS could then re-classify all your properties as dealer status and retroactively tax you at higher and different rates?

Then for holding the propertyies you can do an LLC or just put in personal name and get good liability insurance since an LLC is mainly for liability and can be pierced by good attorneys  Certain things cannot be written off as a landlord/sole proprietor...  

However, there is an out in the law that does not require sole proprietors or owners of a contractor to carry workers comp if they have no employees and can instead opt to pay for his own injuries.

Here is a great article I read recently that explains the massive hypercomplex unpredictability of lawsuits and how having an LLC might not even protect you. http://www.johntreed.com/blogs/john-t-reed-s-real-estate-investment-blog/65996803-how-should-you-hold-title-to-real-estate-or-your-businessIn the end I would suggest staying sole proprietor. 

For instance, if you have a property management company that operates as a sole proprietor.  

Should I remain Sole Proprietor?

@Fred LutesIf you are a sole proprietor, you can deduct your solo 401k contribution on line 28 of Form 1040 which is the adjusted gross income section.

Any sole-proprietor or someone with 1099 income (even part time) can adopt a Solo 401k plan.The UBIT (Unrelated Business Income Tax) exemption only applies on leveraged real estate, whereas in IRA it would be subject to taxes.