Fitness landscapes which include neutrality have been conceptualized as containing neutral networks. Since the introduction of this concept, EC researchers have expected that a population can move along neutral networks without getting trapped on local optima. On the other hand, it has been demonstrated in tunably neutral NK landscapes that neutrality does not affect the ruggedness, although it does reduce the number of local optima. These show that the effects of neutrality are still contentious issues. This paper investigates the effects of neutrality and ruggedness on structures of fitness landscapes. A neutral network is described in a mathematical form based on Harvey's original definition with minor modifications. According to this description, genotypic search space in a problem which includes both neutrality and ruggedness can be classified into several neutral networks. These structures in and between neutral networks are then analyzed. Our results demonstrate that landscapes with a higher degree of neutrality have the larger sizes of neutral networks. For landscapes with the smallest degree of ruggedness, all neutral networks have some contact points to the networks of higher fitness. For landscapes with a higher degree of ruggedness, there are few contact points between the networks of high fitness and the ones of the highest fitness, which seem to be isolated, deceptive or rugged.