diff --git a/lib/if.h b/lib/if.h
index 9796a4d..862f7d4 100644
--- a/lib/if.h
+++ b/lib/if.h
@@ -282,6 +282,7 @@
   u_char flags;
 #define ZEBRA_IFA_SECONDARY    (1 << 0)
 #define ZEBRA_IFA_PEER         (1 << 1)
+#define ZEBRA_IFA_UNNUMBERED   (1 << 2)
   /* N.B. the ZEBRA_IFA_PEER flag should be set if and only if
      a peer address has been configured.  If this flag is set,
      the destination field must contain the peer address.