WIN32_LEAN_AND_MEAN

The Winsock2.h header file internally includes core elements from the
Windows.h header file, so there is not usually an #include line for the
Windows.h header file in Winsock applications. If an #include line is needed for the
Windows.h header file, this should be preceded with the #define WIN32_LEAN_AND_MEAN macro. For historical reasons, the
Windows.h header defaults to including the Winsock.h header file for Windows Sockets 1.1. The declarations in the
Winsock.h header file will conflict with the declarations in the Winsock2.h header file required by Windows Sockets 2.0. The WIN32_LEAN_AND_MEAN macro prevents the
Winsock.h from being included by the Windows.h header.