In a large enough saucepan, add the water and the kombu pieces. On medium heat, slowly bring to almost a boil. Skim any residue or foam as needed. Turn off heat as you start to see bubbles. Remove kombu with tongs and save for other use or discard.
To the saucepan add the dried bonito flakes, stir through and bring to a boil. Once it starts to boil, remove from heat and let steep for 10 minutes.
Strain the dashi through a fine-mesh sieve over measuring bowl. Press wet bonito flakes to extract as much liquid as possible. Discard or save the katsuobushi for other use.
Use right away for soup base of choice or store in the refrigerator covered for 5 days or in the freezer for up to 2 weeks.