I'm using a RFP30N06LE.
This is the circuit:


I'm doing some measurement tests to see if I understood the operation of a mosfet as a switch.

The problem is that both with Vgs=5V (actually 4.7V measured) and with Vgs=0 I have at the ends of resistor R3 about 12V.
Why?
The mosfet should behave like a switch: ideally a short circuit when Vgs=5V (so about 12V on R3) and an open circuit when Vgs=0V (0V on R3)

 

Another problem caused by intermittent breadboard contacts. Solder the simple circuit.

 

I have at the ends of resistor R3 about 12V.

What does this mean? 12V across the resistor, or 12V on each side of the resistor?

 

Your diagram and schematic look correct are you sure you are measuring from the correct end of the resistor, because one end will always be 12 volts.

(Relative to ground)

 

What does this mean? 12V across the resistor, or 12V on each side of the resistor?

I finally solved it by soldering everything on thousand-hole board