+
+unsigned in_addr_netmask_to_prefixlen(const struct in_addr *addr) {
+ assert(addr);
+
+ return 32 - u32ctz(be32toh(addr->s_addr));
+}
+
+int in_addr_default_prefixlen(const struct in_addr *addr, unsigned char *prefixlen) {
+ uint8_t msb_octet = *(uint8_t*) addr;
+
+ /* addr may not be aligned, so make sure we only access it byte-wise */
+
+ assert(addr);
+ assert(prefixlen);
+
+ if (msb_octet < 128)
+ /* class A, leading bits: 0 */
+ *prefixlen = 8;
+ else if (msb_octet < 192)
+ /* class B, leading bits 10 */
+ *prefixlen = 16;
+ else if (msb_octet < 224)
+ /* class C, leading bits 110 */
+ *prefixlen = 24;
+ else
+ /* class D or E, no default prefixlen */
+ return -ERANGE;
+
+ return 0;
+}
+
+int in_addr_default_subnet_mask(const struct in_addr *addr, struct in_addr *mask) {
+ unsigned char prefixlen;
+ int r;
+
+ assert(addr);
+ assert(mask);
+
+ r = in_addr_default_prefixlen(addr, &prefixlen);
+ if (r < 0)
+ return r;
+
+ assert(prefixlen > 0 && prefixlen < 32);
+
+ mask->s_addr = htobe32((0xffffffff << (32 - prefixlen)) & 0xffffffff);
+
+ return 0;
+}