8051 outputs are open drain so when you write a "1" to a port pin then simillar to a open collector output the output pins simply float as the output transistor is off.
Only P0 is open drain.
the output voltage goes high due to the pullup resistors from Vcc connected to the output pin.
Only P1-p3 has pullup
You need to read the 8051 bible links given by erik
Evidently, so do you Prahlad Purohit.

Erik