The ideal body weight chart on the main page of this site were taken by Metropolitan Life and are findings of the weights with the lowest mortality rate based upon the gender, frame size, age and height of the subjects. Just because you are under your ideal weight does not necessarily make you underweight and if you are over your "ideal" weight does not necessarily mean that you are overweight.
An example of someone who is over their "ideal" weight is a bodybuilder. Bodybuilders tend to weigh more than their ideal weight for their height and they are in excellent shape and have a low percentage of body fat. Bodybuilders tend to have a large ammount proportion of muscles, which weigh more than fat.
More important than reaching your ideal weight is mantaining a healthy body fat percentage.