[H] Private IP classes

[DHSinclair]

OK, I now use what I read is the Class C private IP address series.
I use 192.168.2xx.x.  I am told that my sub-net mask should
be 255.255.255.0.
If this is true, then should not my chosen 3rd quartile of "2xx" really
be a value 0?
Or, does it really mean that my chosen 3rd quartile is somehow
ignored?   Yes, I do recall discussion about this last year, but back
then I was using the Class A private IP series......... :)
Thank you for any/all de-mystification.
Best,
Duncan