Automatically assigning an ID number

Automatically assigning an ID number

I'm creating a property management database and need to assign an ID number to each property acquired. I'd like the ID to be automatically assigned so that users don't have to input an ID and so that they can't adjust the ID. I'd like the first property to have an ID of 100, then the next to be assigned 101, then 102, and so on. Does anybody know how to do this?