diff --git a/nest/config.Y b/nest/config.Y
index 792012e7..7bb05259 100644
--- a/nest/config.Y
+++ b/nest/config.Y
@@ -181,8 +181,8 @@ iface_patt_node_init:
 
 iface_patt_node_body:
    TEXT { this_ipn->pattern = $1; this_ipn->prefix = IPA_NONE; this_ipn->pxlen = 0; }
- | prefix { this_ipn->pattern = NULL; this_ipn->prefix = $1.addr; this_ipn->pxlen = $1.len; }
- | TEXT prefix { this_ipn->pattern = $1; this_ipn->prefix = $2.addr; this_ipn->pxlen = $2.len; }
+ | prefix_or_ipa { this_ipn->pattern = NULL; this_ipn->prefix = $1.addr; this_ipn->pxlen = $1.len; }
+ | TEXT prefix_or_ipa { this_ipn->pattern = $1; this_ipn->prefix = $2.addr; this_ipn->pxlen = $2.len; }
  ;
 
 iface_negate:
diff --git a/nest/iface.c b/nest/iface.c
index a80e9736..4d0cf04c 100644
--- a/nest/iface.c
+++ b/nest/iface.c
@@ -588,12 +588,20 @@ iface_patt_match(struct iface_patt *ifp, struct iface *i, struct ifa *a)
 	    continue;
 	}
 
-      // FIXME there should be check for prefix in prefix. (?)
-      if (p->pxlen)
-	if (!a || !ipa_in_net(a->ip, p->prefix, p->pxlen))
-	  continue;
+      if (p->pxlen == 0)
+	return pos;
 
-      return pos;
+      if (!a)
+	continue;
+
+      if (ipa_in_net(a->ip, p->prefix, p->pxlen))
+	return pos;
+
+      if ((a->flags & IA_UNNUMBERED) &&
+	  ipa_in_net(a->opposite, p->prefix, p->pxlen))
+	return pos;
+	  
+      continue;
     }
 
   return 0;