diff --git a/libifupdown/interface.c b/libifupdown/interface.c
index 4035ff5..4f5eafb 100644
--- a/libifupdown/interface.c
+++ b/libifupdown/interface.c
@@ -84,7 +84,7 @@ count_set_bits(const char *netmask)
 }
 
 bool
-lif_address_format_cidr(struct lif_interface *iface, struct lif_dict_entry *entry, char *buf, size_t buflen)
+lif_address_format_cidr(const struct lif_interface *iface, struct lif_dict_entry *entry, char *buf, size_t buflen)
 {
 	struct lif_address *addr = entry->data;
 	size_t orig_netmask = addr->netmask;
diff --git a/libifupdown/interface.h b/libifupdown/interface.h
index 0b57b6c..c86efdc 100644
--- a/libifupdown/interface.h
+++ b/libifupdown/interface.h
@@ -67,7 +67,7 @@ struct lif_interface {
 
 extern bool lif_address_parse(struct lif_address *address, const char *presentation);
 extern bool lif_address_unparse(const struct lif_address *address, char *buf, size_t buflen, bool with_netmask);
-extern bool lif_address_format_cidr(struct lif_interface *iface, struct lif_dict_entry *entry, char *buf, size_t buflen);
+extern bool lif_address_format_cidr(const struct lif_interface *iface, struct lif_dict_entry *entry, char *buf, size_t buflen);
 
 extern void lif_interface_init(struct lif_interface *interface, const char *ifname);
 extern bool lif_interface_address_add(struct lif_interface *interface, const char *address);