Numbers only Textbox

Question

Hi everyone.
I have actually managed to make it so the user can only enter numbers 0-9 and backspace (incase they want to delete the number entered) in the textbox.
I want to know how I can limit the user to only entering numbers between 0-255. I have changed textbox property MaxLength to 3, but the user can still  enter numbers from 0-999.
Thanks in advance.
George.