Every computer on the Internet has an IP address. Your computer, your web enabled phone and the server that hosts your websites all have IP addresses. They usually look something like this:
As you see above, there are 4 sets of numbers separated by dots.
The first set of numbers is the ‘A’ class
The second set is the ‘B’ class
The third set of numbers the ‘C’ class
And finally the fourth set is the ‘D’ class
Something a bit like this: