+unsigned char in_addr_netmask_to_prefixlen(const struct in_addr *addr);
+struct in_addr* in_addr_prefixlen_to_netmask(struct in_addr *addr, unsigned char prefixlen);
+int in_addr_default_prefixlen(const struct in_addr *addr, unsigned char *prefixlen);
+int in_addr_default_subnet_mask(const struct in_addr *addr, struct in_addr *mask);
+int in_addr_mask(int family, union in_addr_union *addr, unsigned char prefixlen);