作用：

Netmask：255.255.255.0 

互转

Prefix：24

新实现：

            static int                                                          NetmaskToPrefix(unsigned char* bytes, int bytes_size) noexcept {if (NULL == bytes || bytes_size < 1) {return 0;}int prefix = 0;for (int i = 0; i < bytes_size; i++) {int b = bytes[i];while (b) {prefix += b & 1; b >>= 1;}}return prefix;}

源实现：

            static UInt32                                                       PrefixToNetmask(int prefix) noexcept {UInt32 mask = prefix ? (-1L << (32L - prefix)) : 0L;return htonl(mask);}static int                                                          NetmaskToPrefix(UInt32 mask) noexcept {return NetmaskToPrefix(reinterpret_cast<unsigned char*>(&mask), sizeof(mask));}static int                                                          NetmaskToPrefix(unsigned char* bytes, int bytes_size) noexcept {int bitLength = 0;int idx = 0;if (NULL == bytes || bytes_size < 0) {return bitLength;}// find beginning 0xFFfor (; idx < bytes_size && bytes[idx] == 0xff; idx++);bitLength = 8 * idx;if (idx < bytes_size) {switch (bytes[idx]) {case 0xFE: bitLength += 7; break;case 0xFC: bitLength += 6; break;case 0xF8: bitLength += 5; break;case 0xF0: bitLength += 4; break;case 0xE0: bitLength += 3; break;case 0xC0: bitLength += 2; break;case 0x80: bitLength += 1; break;case 0x00: break;default: // invalid bitmaskreturn ~0;}// remainder must be 0x00for (int j = idx + 1; j < bytes_size; j++) {unsigned char x = bytes[j];if (x != 0x00) {return ~0;}}}return bitLength;}