diff --git a/iputil.rkt b/iputil.rkt
index 02664df..ecc9474 100644
--- a/iputil.rkt
+++ b/iputil.rkt
@@ -44,9 +44,10 @@
   (string-join parts "."))
 
 (define (ip-netmask ip)
-  (for/sum ([i (in-range 32)])
-    #:break (positive? (bitwise-and ip (arithmetic-shift 1 i)))
-    1))
+  (- 32
+     (for/sum ([i (in-range 32)])
+       #:break (positive? (bitwise-and ip (arithmetic-shift 1 i)))
+       1)))
 
 (define (string->subnet str)
   (match-define (list ipstr maskstr) (string-split str "/"))