Yes the default IP address of an M340 PLC can be determined by converting the last 3 octets in the MAC address, setting the rotary switch to Clear IP and preceding the IP address with 84.
Note: In order to write a new IP address to the M340 after the configuration is saved to flash the rotary switch needs to go back to stored.

Example: If the MAC address is 00.80.F4.02.EA.6A then the IP address will be 84.2.234.106

02 Hex = 02 Decimal
EA Hex = 234 Decimal
6A Hex = 106 Decimal