Please follow these steps:
1) Press the CONFIG key seven times.
2) Turn the VOLTAGE knob to select CF61.
3) Turn the CURRENT knob to set the address assignment to MANUAL (001).
4) Set the IP address using CF62 through CF65.
5) Set the subnet mask using CF66. The subnet mask designation should be a value between 1 and 31. For example, if you want to set a subnet mask of “255.255.0.0,” set it to 16.

Please note that each setting takes effect when the POWER switch is turned off and on again.
For the details, read the user’s manual – chapter of Basic Features – CONFIG Settings.