What is meant by fixed length sub-net masking? Is it mean Class A -----255.0.0.0 Class B -----255.255.0.0 Class C -----255.255.255.0
As i understand its mean is keep the length of the network prefix length constant. Is it right?
Yes, but most importantly, classful addressing (i.e. fixed length subnet masks) is obsolete and has been for at least 20 years. I don't know why networking classes still teach it. There are many, many resources on this and other stack exchanges (as well as the Internet) that explain how to use variable length masks.