In small networks, all systems are connected to the same network cable:

The communication between any 2 systems uses the standard components of Windows Networking:

When working on system#1 (using the Network CLIENT) and accessing data on System#2 (made available via a Network SERVER) , the connection is made via the Network-protocols and Network Adapters on both systems.

But networks grow, requiring the use of MULTIPLE Network cables or the need for different types of Network Media (like: modems, ISDN,....):

Now, some systems (like #2 above) have
multiple Network Adapters and any communications from system#1 to #3 and #4 has to go via System#2, which has now the additional job of forwarding/passing on Network communication.
Such a forwarding is called : Routing
Now, additional functionality in the Windows Networking is required:

The PROTOCOL component of the Router (#2 above) needs to be able to handle this forwarding. Now the proper
selection of the Network protocol is important:
- NetBEUI is NOT able to route and CANNOT be used in such configurations
IPX/SPX and TCP/IP allow routing.

There can be multiple Routing on Networks:

and when using TCP/IP on the Internet, there can be 10 or more routers involved for a connection to a specific site.

Setup TCP/IP Routing