Modbus TCP/IP: - only physical layer and data link layer changers but original communication standard (Modbus )is as it is, i.e., data frame is having 64 octates but after finishing Modbus data other octates sent blank.
In Ethernet TCP/IP: - it is full of data ..64 octates with CRC party Strat & stop bits.
MODBUS is a communication protocol (serial) created a long time ago. With the advent of TCP / IP (transport protocol and Internet protocol) the industry has developed a MODBUS messaging service over
TCP/IP which then enables all legacy modbus devices and new devices to continue to communicate over the TCP/IP "channel".
There are more detailed documents on the MODBUS protocol on the Internet.
If you knowledge is deeper than what I have tried to explain above then you may need to refer to device specific tutorials on how to configure the Modbus signaling protocol to work over an Ethernet backbone.
Another way of thinking about this is in a simplified way:
Ethernet - Physical layer standard describing wiring and electrical signals (e.g. voltage, timing). Think of this as the interstate highway.
TCP/IP - Logical layer (Protocol) that describes how to move information across the Ethernet network - (e.g. packet structure, IP addressing). Think of this as a truck with directions to get from point A to point B.
Modbus TCP/IP - This is the definition of how to structure data that can be read by devices (SCADA/PLC/RTU). The data is carried by TCP/IP and Ethernet between two systems. Think of this as cargo being carried by the truck between point A and B.