In the computer networks course, we solve subnetting questions together with the teacher. There is a term we call the "main network". As far as I understand, we fill in this according to whether the IP address is A, B or C class. But since classful IP addresses are no longer used today, what exactly does main network mean today? If it makes any sense, for example, how to find the main network of an IP address? I wanted to ask because it confused me as a student. Thank you in advance for your answers.
This is an example we used in the lesson: https://web.archive.org/web/20080405113738/http://www.brouckie.be/cgi-bin/IPsubnetting/index.pl