diff --git a/sys/net/application_layer/dhcpv6/client.c b/sys/net/application_layer/dhcpv6/client.c
index b33aeaa528..8d7471d7a8 100644
--- a/sys/net/application_layer/dhcpv6/client.c
+++ b/sys/net/application_layer/dhcpv6/client.c
@@ -15,6 +15,7 @@
 
 #include <assert.h>
 #include <stdbool.h>
+#include <string.h>
 
 #include "event.h"
 #include "event/timeout.h"
@@ -448,11 +449,11 @@ static inline size_t _compose_iapfx_opt(dhcpv6_opt_iapfx_t *iapfx,
     if (lease->pfx_len == 0) {
         return 0;
     }
+    /* set all unused/requested fields to 0 */
+    memset(iapfx, 0, sizeof(*iapfx));
 
     iapfx->type = byteorder_htons(DHCPV6_OPT_IAPFX);
     iapfx->len = byteorder_htons(len);
-    iapfx->pref = byteorder_htonl(0);
-    iapfx->valid = byteorder_htonl(0);
     iapfx->pfx_len = lease->pfx_len;
 
     return len + sizeof(dhcpv6_opt_t);
@@ -505,28 +506,33 @@ static inline size_t _add_ia_pd_from_config(uint8_t *buf, size_t len_max)
     if (!IS_USED(MODULE_DHCPV6_CLIENT_IA_PD)) {
         return 0;
     }
-
-    size_t msg_len = 0;
+    uint8_t *start = buf, *end = &buf[len_max];
 
     for (unsigned i = 0; i < CONFIG_DHCPV6_CLIENT_PFX_LEASE_MAX; i++) {
         uint32_t ia_id = pfx_leases[i].parent.ia_id.id;
         if (ia_id != 0) {
+            size_t len = 0;
+            /* add IA Prefix Option included in IA Prefix Delegation Option */
+            dhcpv6_opt_iapfx_t *pa_pfx = (dhcpv6_opt_iapfx_t *)(end - sizeof(dhcpv6_opt_iapfx_t));
+            if ((uint8_t *)pa_pfx < start) {
+                assert(0); /* not supposed to happen */
+                return 0;
+            }
+            len = _compose_iapfx_opt(pa_pfx, &pfx_leases[i], len);
+            end -= len;
             /* add Identity Association for Prefix Delegation Option */
-            dhcpv6_opt_ia_pd_t *ia_pd = (dhcpv6_opt_ia_pd_t *)(&buf[msg_len]);
-            msg_len += _compose_ia_pd_opt(ia_pd, ia_id, 0);
-
-            /* add IA Prefix Option */
-            dhcpv6_opt_iapfx_t *pa_pfx = (dhcpv6_opt_iapfx_t *)(&buf[msg_len]);
-            msg_len += _compose_iapfx_opt(pa_pfx, &pfx_leases[i], 0);
+            dhcpv6_opt_ia_pd_t *ia_pd = (dhcpv6_opt_ia_pd_t *)(end - sizeof(dhcpv6_opt_ia_pd_t));
+            if ((uint8_t *)ia_pd < start) {
+                assert(0); /* not supposed to happen */
+                return 0;
+            }
+            len = _compose_ia_pd_opt(ia_pd, ia_id, len);
+            /* move to buffer start */
+            memmove(start, ia_pd, len);
+            start += len;
         }
     }
-
-    if (msg_len > len_max) {
-        assert(0);
-        return 0;
-    }
-
-    return msg_len;
+    return start - buf;
 }
 
 static inline int32_t get_rand_ms_factor(void)