Cook sugar, 1/2 cup water, and lemon juice in a small saucepan over medium heat, stirring constantly, 2 minutes or until sugar dissolves. Bring to a boil over medium-high heat; cook, stirring occasionally, 3 minutes. Cool.
Stir together sugar mixture, remaining 6 cups water, and lime juice in a large pitcher; chill. Serve over ice, if desired.