The controllers must send command responses back to your computer. They send these datagrams to your computer's IP address. The controllers need to determine your computer's IP address in order to do this. You can configure the OMRON Ethernet modules to use two different methods: table look-up and automatic address generation. Both use the SA1 value of your computer. SA1 is the FINS value you assign for the Node Number of your computer on the OMRON Ethernet network.
For the table look-up method, the Ethernet Unit simply looks up the SA1 value sent in the original command in a table your define in the controller to find the computer's IP address. Using OMRON programming software, you must enter both the SA1 and IP address values in the table for each OMRON Ethernet module that uses this method.
For the automatic address generation method, the Ethernet modules use the SA1 value to calculate the IP address by performing a logical AND between the Ethernet Module's IP address and the subnet mask, and then adding the FINS node number (SA1).
OMRON TCP/IP application notes.