diff --git a/sys/netinet/in.h b/sys/netinet/in.h
index 01444164028d..a4e9f0141126 100644
--- a/sys/netinet/in.h
+++ b/sys/netinet/in.h
@@ -354,6 +354,7 @@ __END_DECLS
 #define	INADDR_UNSPEC_GROUP	(u_int32_t)0xe0000000	/* 224.0.0.0 */
 #define	INADDR_ALLHOSTS_GROUP	(u_int32_t)0xe0000001	/* 224.0.0.1 */
 #define	INADDR_ALLRTRS_GROUP	(u_int32_t)0xe0000002	/* 224.0.0.2 */
+#define	INADDR_ALLMDNS_GROUP	(u_int32_t)0xe00000fb	/* 224.0.0.251 */
 #define	INADDR_MAX_LOCAL_GROUP	(u_int32_t)0xe00000ff	/* 224.0.0.255 */
 
 #define	IN_LOOPBACKNET		127			/* official! */
diff --git a/sys/netinet6/in6.h b/sys/netinet6/in6.h
index 1a504c8bd788..a1783711bcc4 100644
--- a/sys/netinet6/in6.h
+++ b/sys/netinet6/in6.h
@@ -224,6 +224,9 @@ extern const struct in6_addr in6mask128;
 #define IN6ADDR_LINKLOCAL_ALLROUTERS_INIT \
 	{{{ 0xff, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, \
 	    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02 }}}
+#define IN6ADDR_LINKLOCAL_ALLMDNS_INIT \
+	{{{ 0xff, 0x02, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, \
+	    0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0xfb }}}
 #endif
 
 extern const struct in6_addr in6addr_any;