A routing table defines which PLC communication card is connected to each network. This ensures that communications are sent out through the correct port to reach the intended target device.

A routing table is required in the following situations:

1. When communicating with a PLC or computer on another network (e.g. remote programming or monitoring using FINS messages or a CX-Programmer)
2. When multiple Communication Units are mounted to a single PLC.
   One example where this may not be obvious is when configuring a DeviceNet network while connected to the PLC via FINS Ethernet. In this case, a device on one network (the PC) is communicating with devices on another network (the DeviceNet devices), so a routing table is required.
3. When routing tables are used for one or more other nodes on the same network
4. Individual devices on the network do not support communication on the default network (Network 0).
   For example, an NS HMI communicating via FINS must use a network address of 1 or higher, which requires a routing table to be configured.

It is not necessary to set routing tables if one Communications Unit is mounted to the PLC and the nodes are connected as one network.