Network Operating System

Solution 1:

It's an obsolete term, no one uses it in the context you're asking about any more. I would say that it's been obsolete since the early 90's: somewhere around that time, networking features became a required part of any OS.

Solution 2:

Once upon a time the NOS was distinct from the OS, mainly because networking was added long after computers already had operating systems. As such it was an add-on to the OS. Even as recently as the 80's most computers were sold with no networking capabilities as standard. These days the OS and NOS are intertwined to the point that they are, for all practical purposes, one and the same. So, as Ward pointed out, it's an obsolete term in today's context.