i need explanation, hope and looking for the answers:

Q1.what is meaning of "private" ID with example?

Q2.how can i write private ID for class A,B,C>

Q3.important Q..Host need: mean subnet mask,how can i find it with two digit like example: 20

Q4. Network need: also i think want subnet mask, how can i find it with 5 digit

i will wait a response, and for sure i will give Positive evaluation -_-