diff --git a/iputil.rkt b/iputil.rkt
index e30ee46..1affb98 100644
--- a/iputil.rkt
+++ b/iputil.rkt
@@ -4,6 +4,7 @@
          string->ip
          ip->string
          ip-netmask
+         netmask-ip
          string->subnet
          subnet->string
          string->peer
@@ -51,6 +52,10 @@
        #:break (positive? (bitwise-and ip (arithmetic-shift 1 i)))
        1)))
 
+(define (netmask-ip mask)
+  (arithmetic-shift (sub1 (arithmetic-shift 1 mask))
+                    (- 32 mask)))
+
 (define (string->subnet str)
   (match-define (list ipstr maskstr) (string-split str "/"))
   (subnet (string->ip ipstr)
@@ -78,6 +83,9 @@
   (check-equal? (ip->string (string->ip "123.84.0.67")) "123.84.0.67")
   (check-equal? (ip->string (string->ip "0.0.0.0")) "0.0.0.0")
 
+  (check-equal? (ip->string (netmask-ip 23)) "255.255.254.0")
+  (check-equal? (ip->string (netmask-ip 8)) "255.0.0.0")
+
   (check-equal? (ip-netmask (string->ip "255.255.254.0")) 23)
   (check-equal? (ip-netmask (string->ip "255.0.0.0")) 8)
   (check-equal? (subnet->bl (string->subnet "3.3.0.0/24"))