Place ginger and 4 cups of water in a 4-quart saucepan. Bring to a boil over high heat. Simmer at a low boil, uncovered, for about 20 minutes. Strain into large measuring cup or bowl. Put pulp back in pan, add 2 cups water and simmer again for 20 minutes. Repeat with remaining 2 cups water.

Discard pulp and strain through cheesecloth. Return liquid to pan. Add sugar, stirring constantly, and continue to boil until reduced to about 5 cups.

Cool the syrup, pour into a glass container, seal tightly, and chill at least 1 hour until cold. Store or up to a week. Longer if frozen

For each 16-ounce serving amount, mix 1/4 cup ginger syrup with cold soda. Pour over ice. Additional ginger syrup and/or sugar may be added to taste.