1111 1111 1111 1111 1111 1111 1000 0000

octet 1  .    octet 2  .    octet 3  .    octet 4
0000 0000     0000 0000     0000 0000     0000 0000
1111 1111     1111 1111     1111 1111     1000 0000

255      .    255      .    255      .    128

In bytes:    128.2.19.0
In binary    10000000 00000010 00010011 00000000
The bitmask: 11111111 11111111 11111111 10000000
Ergo:        ------- network ------------  host

#include <stdio.h>
#include <arpa/inet.h>

uint32_t cidr_to_netmask(uint8_t cidr)
{
  uint8_t unset_bits = 32 - cidr;
  return ntohl(0xffffffff << unset_bits);
}

int main(void)
{
    uint8_t cidr = 25;

    uint32_t _netmask = cidr_to_netmask(cidr);
    struct in_addr _netmask_addr = { _netmask };

    char netmask[16];
    if (inet_ntop(AF_INET, (struct in_addr *)&_netmask_addr, (char *)&netmask, sizeof(netmask)) == NULL) {
      fprintf(stderr, "error.n");
      return 1;
    }

    printf("%d = %sn", cidr, netmask);

    return 0;
}

25 = 255.255.255.128