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a guest Feb 17th, 2019 61 Never
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  1. 1111 1111 1111 1111 1111 1111 1000 0000
  2.    
  3. octet 1  .    octet 2  .    octet 3  .    octet 4
  4. 0000 0000     0000 0000     0000 0000     0000 0000
  5. 1111 1111     1111 1111     1111 1111     1000 0000
  6.    
  7. 255      .    255      .    255      .    128
  8.    
  9. In bytes:    128.2.19.0
  10. In binary    10000000 00000010 00010011 00000000
  11. The bitmask: 11111111 11111111 11111111 10000000
  12. Ergo:        ------- network ------------  host
  13.    
  14. #include <stdio.h>
  15. #include <arpa/inet.h>
  16.  
  17. uint32_t cidr_to_netmask(uint8_t cidr)
  18. {
  19.   uint8_t unset_bits = 32 - cidr;
  20.   return ntohl(0xffffffff << unset_bits);
  21. }
  22.  
  23. int main(void)
  24. {
  25.     uint8_t cidr = 25;
  26.  
  27.     uint32_t _netmask = cidr_to_netmask(cidr);
  28.     struct in_addr _netmask_addr = { _netmask };
  29.  
  30.     char netmask[16];
  31.     if (inet_ntop(AF_INET, (struct in_addr *)&_netmask_addr, (char *)&netmask, sizeof(netmask)) == NULL) {
  32.       fprintf(stderr, "error.n");
  33.       return 1;
  34.     }
  35.  
  36.     printf("%d = %sn", cidr, netmask);
  37.  
  38.     return 0;
  39. }
  40.    
  41. 25 = 255.255.255.128
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