Active worms propagate across networks by employing various target discovery techniques. The significance of target discovery techniques in shaping a worm's propagation characteristics is derived from the life cycle of the worm. Various target discovery techniques that could be employed by active worms are discussed and compared. It is anticipated that future active worms would employ multiple target discovery techniques simultaneously to greatly accelerate their propagation. To accelerate a worm's propagation, the slow start phase in the worm's propagation must be shortened by letting the worm infect the first certain percentage of susceptible hosts as soon as possible. Strategies that future active worms might employ to shorten the slow start phase in their propagation are studied. Their respective cost-effectiveness is assessed. A novel active defense mechanism which can effectively defend against the propagation of active worms is proposed.