Remove rib roast from packaging and place in a roasting pan. Let come to room temperature about 30 minutes. Preheat oven to 450º. Generously cover all sides of rib roast with salt and pepper. Place roast in a roasting pan fat-side up. Arrange half the rosemary in the bottom of the pan around the roast.
Roast 30 minutes, then reduce heat to 350º and cook 1 hour 30 minutes more for medium rare. (Plan on about 15 minutes per pound.) Remove roast from oven, cover with foil, and let rest 20 minutes.
To serve, slice along ribs to remove them, then separate each rib and put them aside. Slice the roast crossways against the grain. Serve on a platter with the ribs and remaining half fresh rosemary.