The Ethereum Foundation has stated that Ethereum:

In reality, different parts of the Ethereum technology stack are licensed – or will be licensed—in different ways.
The core parts of the stack, including the consensus engine, networking code, and supporting libraries, haven't yet been licensed, but are expected to be covered by either the MIT, MPL, or LGPL license. It's important to understand that the first of these is a permissive license, while the latter two are more restrictive, and restrict a user's ability to distribute any modifications under commercial terms, therefore potentially restricting how businesses can use the code.

Regardless of the potential future restrictions that may appear once licensing is confirmed, several enterprise-centric implementations of Ethereum have already been created, with two such implementations being Quorum, created by J.P.Morgan, and Monax. Later in the chapter we will be discussing and implementing a project based on Quorum.