diff --git a/lib/ipv4.h b/lib/ipv4.h
index de9f4e57..add406b0 100644
--- a/lib/ipv4.h
+++ b/lib/ipv4.h
@@ -51,6 +51,7 @@ typedef u32 ip_addr;
 #define ipa_hton(x) x = _MI(htonl(_I(x)))
 #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)
 
 int ipv4_classify(u32);