During regular volleys, any part of a player's body may make come in contact with the ball, so a kicked ball or even a ball that hits a player's foot and rebounds is legal. On the first contact, such as defensively playing a hard-driven spike, the ball may make double contact, such as with a player's foot and hand, as long as the referee deems the double contact to be occurring during a single action.
Indoor Versus Outdoor
In beach games, a player may go completely under the net and into the opponent's court, as long as she does not interfere with the opponent; however, in indoor games, a player's foot or feet must be on or above the center line that runs parallel to and directly below the net. In other words, she may go completely into the opposing team's court as long as her feet make contact with or are directly above the center line. Neither foot may cross completely.
Beach players play barefoot unless socks are authorized by the referee. According to the FIVB, indoor players may not play barefoot in the Olympics or in world and official competitions that are played indoors.
During a serve, a player may not step on the baseline or into the court until after the ball has been struck and is in flight. For a jump serve, the player must leave the ground before the baseline and strike the ball before landing inside the court. A player who steps on the baseline or into the court before the serve is released has committed a foot fault. His team loses a point and the serve.
As of 2008, players in sanctioned competition can make contact with the net with any part of their body, including their feet, as long as the contact is not in the act of making contact with the net or interferes with the play of the opponent.