Programlisting of only part of a file

All,
I have looked for this answer. I think that I remember someone asking this but I cannot find it in the forum.
 
How do I <programlisting> to show only part of a file? Assuming the file was 100 lines lond and I only want to show a snippet of code - 5 lines long starting at line 20.
 
Cheers!
 
Dean Nelson