In order for Windows 3.1 to use networking, a DOS NIC driver, protocol, and client software had to be provided. As networking software became extremely feature rich, the size of the client software resident in the real-mode (640k) portion of RAM grew to to the point that many applications would not run due to insufficient free real-mode RAM.
Windows For Workgroups solved this problem by adding protected mode network support. Rather than loading drivers in to the 640K segment, the Microsoft Windows Network software loads in the form of ".386" files (VxD files).
