diff --git a/lib/ipv4.c b/lib/ipv4.c
index 9aa79c26..5ec2ffb8 100644
--- a/lib/ipv4.c
+++ b/lib/ipv4.c
@@ -43,11 +43,26 @@ ip_ntop(ip_addr a, char *b)
 		      (x & 0xff));
 }
 
-
 char *
 ip_ntox(ip_addr a, char *b)
 {
   return b + bsprintf(b, "%08x", _I(a));
 }
 
+u32
+ipv4_class_mask(u32 a)
+{
+	u32 m;
+
+	if (a < 0x80000000)
+		m = 0xff000000;
+	if (a < 0xc0000000)
+		m = 0xffff0000;
+	else
+		m = 0xffffff00;
+	while (a & ~m)
+		m |= m >> 1;
+	return m;
+}
+
 #endif
diff --git a/lib/ipv4.h b/lib/ipv4.h
index e38dce6b..a7715001 100644
--- a/lib/ipv4.h
+++ b/lib/ipv4.h
@@ -54,8 +54,10 @@ typedef u32 ip_addr;
 #define ipa_ntoh(x) x = _MI(ntohl(_I(x)))
 #define ipa_classify(x) ipv4_classify(_I(x))
 #define ipa_opposite(x) _MI(_I(x) ^ 1)
+#define ipa_class_mask(x) x = _MI(ipv4_class_mask(_I(x)))
 
 int ipv4_classify(u32);
+u32 ipv4_class_mask(u32);
 
 /* FIXME: Is this hash function uniformly distributed over standard routing tables? */
 static inline unsigned ipv4_hash(u32 a)