To implement a TCP/IP network, you need to be assigned a formal IP range from
the Internet community. However, since you probably plan to create a "private"
Modbus/TCP network without direct access from the world, there are official
guidelines for how to select your own IP. If you follow these guidelines (called RFC1918),
then you neither need to register nor risk conflicting with others.
RFC1918 is abit like the spread-spectrum of Internet addressing!
to view a PDF file.