diff --git a/lib/isc/include/isc/netaddr.h b/lib/isc/include/isc/netaddr.h
index b252432482..f6e569e34b 100644
--- a/lib/isc/include/isc/netaddr.h
+++ b/lib/isc/include/isc/netaddr.h
@@ -77,6 +77,12 @@ isc_netaddr_format(isc_netaddr_t *na, char *array, unsigned int size);
  * The resulting string is guaranteed to be null-terminated.
  */
 
+#define ISC_NETADDR_FORMATSIZE \
+	sizeof("xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:XXX.XXX.XXX.XXX")
+/*
+ * Minimum size of array to pass to isc_netaddr_format().
+ */
+
 void
 isc_netaddr_fromsockaddr(isc_netaddr_t *netaddr, const isc_sockaddr_t *source);
 
diff --git a/lib/isc/include/isc/sockaddr.h b/lib/isc/include/isc/sockaddr.h
index cf4476984c..781259ae3b 100644
--- a/lib/isc/include/isc/sockaddr.h
+++ b/lib/isc/include/isc/sockaddr.h
@@ -121,6 +121,9 @@ isc_sockaddr_format(isc_sockaddr_t *sa, char *array, unsigned int size);
 
 #define ISC_SOCKADDR_FORMATSIZE \
 	sizeof("xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:XXX.XXX.XXX.XXX#YYYYY")
+/*
+ * Minimum size of array to pass to isc_sockaddr_format().
+ */
 
 ISC_LANG_ENDDECLS