The DN code is a code that rounds off the diameter of the pipe to get an even number to work with, not the exact diameter. The American version is called NPS and is in english units. DN is the european version and is based on millimeters. For example, If you want 2" Pipe, NPS calls it 2" and DN calls it 50 mm. but the actual outside diameter is neither 2" or 50 mm. They just use these nice round numbers cause it is easier.

The term 'Nominal diameter' as it applies to the area of reclamation can be defined as ' An approximate measurement of the diameter of a pipe. Although the nominal diameter is used to describe the size or diameter of a pipe, it is usually not the exact inside diameter of the pipe'.