Alternatively, if the network supports DHCP, you can enable it (click DHCP Enabled). In this case, the IP Address and Subnet Mask fields become read only.
Note that in general (for stability, static IP addressing is recommended over DHCP. If DHCP is preferred, an IP Address 'Reservation' should be entered for the controller in the DHCP Server. The controller IP address should not change.
In any case, note that only one LAN port can be set as a DHCP server. If enabling LAN2, you must specify another (network) static IP address and the appropriate subnet mask, i.e. a different subnet mask for each enabled LAN port IP address.