There is no “Inner” and “Outer” solar system with a hard defined boundary. Generally the solar system is divided into the rocky planets (which ends at Mars), the gas giants (which starts at Jupiter), and the Kuiper Belt (which is all of the objects beyond the orbit of Neptune).

So you would just say the asteroid belt is the region of space between the rocky planets and the gas giants, or between Mars and Jupiter. There is no real agreement in what the “inner” and “outer” solar system is or if they are even fair classifications. Especially now that the Kuiper Belt is so frequently referred to as the outer solar system by laymen.