"I live in Milan, Via Matteotti 18" is more correct.One would use the preposition "on" instead of "in" when referring to a street. However, when referring to a room number in a hotel or apartment, you would use "in". Here are a few more examples for reference: I live in Chicago (city), on Grant Street. I live on Clover Street, in Apartment #120. I am staying in the Fairmont Hotel, in room 11. Another way is to simply state your address: My address is 1234 Apple Street, in Atlanta.