Question: why 1 byte is 255?
Answer: A byte has only 8 bits. A bit is a binary digit. So a byte can hold 2 (binary) ^ 8 numbers ranging from 0 to 2^8-1 = 255. It's the same as asking why a 3 digit decimal number can represent values 0 through 999, which is answered in the same manner (10^3 - 1).