diff --git a/config.h b/config.h
index a56d6fc..05b3a5a 100644
--- a/config.h
+++ b/config.h
@@ -21,7 +21,7 @@
 #include <netinet/in.h>
 
 #define DEFAULT_IPV4_LOCAL_SUBNET "192.0.0.4"
-
+#define DEFAULT_IPV4_LOCAL_PREFIXLEN "29"
 #define DEFAULT_DNS64_DETECTION_HOSTNAME "ipv4only.arpa"
 
 struct clat_config {
@@ -29,6 +29,7 @@
   struct in6_addr ipv6_local_subnet;
   struct in6_addr ipv6_host_id;
   struct in_addr ipv4_local_subnet;
+  int16_t ipv4_local_prefixlen;
   struct in6_addr plat_subnet;
   char *default_pdp_interface;
   char *plat_from_dns64_hostname;
@@ -39,6 +40,9 @@
 int read_config(const char *file, const char *uplink_interface, const char *plat_prefix,
         unsigned net_id);
 void config_generate_local_ipv6_subnet(struct in6_addr *interface_ip);
+in_addr_t config_select_ipv4_address(const struct in_addr *ip, int16_t prefixlen);
 int ipv6_prefix_equal(struct in6_addr *a1, struct in6_addr *a2);
 
+typedef int (*addr_free_func)(in_addr_t addr);
+
 #endif /* __CONFIG_H__ */