diff --git a/lib/prefix.h b/lib/prefix.h
index 0546095..7ef5347 100644
--- a/lib/prefix.h
+++ b/lib/prefix.h
@@ -173,6 +173,8 @@
 void masklen2ip6 (int, struct in6_addr *);
 
 void str2in6_addr (const char *, struct in6_addr *);
+const char *inet6_ntoa (struct in6_addr *);
+
 #endif /* HAVE_IPV6 */
 
 int all_digit (const char *);