Microsoft released Version 4.0 under an Apache 2.0 license. The .NET Micro Framework is a “development and execution environment for resource-constrained devices.” .NET is a generalized development system (including standard libraries) for Microsoft platforms, and resource-constrained devices mostly means embedded systems or single-purpose devices like cable boxes, medical devices or kiosks. The .NET Micro Framework enables development of .NET applications on these devices.
TCP/IP stack and cryptography stack were omitted from the open source release. The TCP/IP stack was third party software not available for open source release, and the cryptography stack was described as replaceable.