I was confused with this answer because I have the same issue as original question i wanted default value 00....but you mentioned like we need to enable pull up.
The suggestion to use pull-ups was so that you had valid inputs. Without an active sensor, pull-up or pull-down, the input pin(s) are floating and are invalid, unknown logic levels.
Why do you need inputs to be zero? You’ve not explained anything about your inputs other than you’re using 3 volts to test. Is that the supply voltage of the MCP23017? Is that the system voltage?
i have to enable pull down for all 16 additional gpios by adding resistor between pin and ground,right?
Yes, if you need pull-downs for some unknown, unexplained reason, you must must add external resistors. Most times though, you would just enable the internal pull-up and you’re done. If you’re reading a switch and you want the logic reversed so the the input is a zero with the button released and a one with the button pressed, let the compiler do it for you by reading the input like this:
inputPushbutton = ! digitalRead(pin);