diff --git a/sys/netinet/in.c b/sys/netinet/in.c
--- a/sys/netinet/in.c
+++ b/sys/netinet/in.c
@@ -441,6 +441,27 @@
 	return (error);
 }
 
+int
+in_mask2len(struct in_addr *mask)
+{
+	int x, y;
+	u_char *p;
+
+	p = (u_char *)mask;
+	for (x = 0; x < sizeof(*mask); x++) {
+		if (p[x] != 0xff)
+			break;
+	}
+	y = 0;
+	if (x < sizeof(*mask)) {
+		for (y = 0; y < 8; y++) {
+			if ((p[x] & (0x80 >> y)) == 0)
+				break;
+		}
+	}
+	return (x * 8 + y);
+}
+
 int
 in_control(struct socket *so, u_long cmd, void *data, struct ifnet *ifp,
     struct thread *td)
diff --git a/sys/netinet/in_var.h b/sys/netinet/in_var.h
--- a/sys/netinet/in_var.h
+++ b/sys/netinet/in_var.h
@@ -459,6 +459,7 @@
 int	in_leavegroup(struct in_multi *, /*const*/ struct in_mfilter *);
 int	in_leavegroup_locked(struct in_multi *,
 	    /*const*/ struct in_mfilter *);
+int	in_mask2len(struct in_addr *);
 int	in_control(struct socket *, u_long, void *, struct ifnet *,
 	    struct thread *);
 int	in_control_ioctl(u_long, void *, struct ifnet *,