DN (i.a.w. ISO 6708) is an alphanumeric identifier of size for components in a piping system and is used for reference purposes. It comprises the letters DN, followed by a dimensionless value (e. g. DN 100) that is indirectly related to the physical size of the hole or outside diameter of the connections (in millimetres), on piping, valves, and nozzles of pump casings (also see Nominal diameter). Further identifiers apply to products with threads or cutting ring connections as well as soldered or welded connections and products and include NPS (Nominal Piping Size), OD (Outside Diameter) and ID (Inside Diameter) (EN 1759 or ASME B 16.5).
Fig. 1 DN provides an overview of preferred DN values in accordance with ISO 6708 (DIN 2402).