In a large saucepan, combine 6 cups of sugar, 2 cups of water, and 3 inches of peeled and sliced ginger.
Bring the mixture to a boil, stirring until the sugar is dissolved.
Simmer over low heat, stirring occasionally for 15 minutes.
Stir in 1 1/2 cups of lime juice.
Simmer the mixture until syrupy, about 15 minutes.
Strain the syrup into jars.
Let the syrup cool completely before storing.
The syrup can be refrigerated for up to 1 month.