After the physical network the NOS, Network Operating System, is the most important part in a PC network. The network operating system is responsible for the levels 3 to 7 in the OSI model. This means that NOS is independent of the type of network. It works in the same manner on an Ethernet as a Token Ring.
There are two types of NOS. The first type is called "peer to peer” which means that no dedicated server is needed. The other type is server based and is used for larger, more demanding networks.
There are many vendors with their own types of NOS on the market.
The leading systems for "peer to peer” based NOS are NetWare Lite and Windows for Workgroups.
The leading systems for server based NOS are Novell NetWare, Windows NT and OS/2 LAN Server.