Subnets that need routing to 

System->SubNets

If your network is comprised of several subnets, rather than one single physical network, you need to enter the details of those subnets, and how to route network traffic to them, into this table, before you can connect with any computers on those subnets.

If you only have a single physical network, with no Routers to direct TCP/IP traffic between separate segments, then you do not need to make any entries in this table.

The three pieces of information that are required in order to route network traffic to a computer on a subnet are:

IP Address The Network Address of the subnet, which can be calculated by AND-ing the IP address of any machine on the subnet, with its network mask.
Netmask The Network Mask of the subnet, which defines how many host addresses are available on the subnet.
Gateway This is the local IP address of the gateway router that is used to direct network packets to the subnet.

An example of when this might be applicable is where a server has two network adapters, which means it will probably have two different subnets (sets of IP addresses) assigned to the LANs that are attached to the two network adapters. If a computer on one LAN wants to send information on to a computer on the other LAN it must send the information via the Server which will forward the information to the other network. To do this it must be told that the second LAN is only accessible via the LAN.

Similarly, if the Server is on the first LAN it must be told that the computers on the second LAN can only be reached via the existing Server.  This kind of setup is quite common where a company is using a Novell Server with TCP/IP networking.