Add water to a 2-quart saucepan.
Check kombu and wipe clean any dirt or grit (usually not needed for today's commercial preparations.) DO NOT wipe off white substance (mannitol) as it contributes to the umami. Make a couple slits in the kombu to help release more favor.
Put kombu into the water to rehydrate for a minimum of 2 to 3 hours in the summertime or 4 to 5 hours in the winter time. It is best though if left to brew overnight.
Slowly bring kombu dashi to a boil over medium-low heat. Just before the dashi begins to boil, remove the kombu. If left in the kombu becomes slimy and the dashi, bitter.
Stop here if you are vegetarian/vegan.