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

#define uint8_t unsigned char
#define uint32_t unsigned int

int main(int argc,char **argv)
{
    // 1.1.0.0/16的网段 
    uint32_t ipv4 = 0x1010000;
    uint8_t  prefix = 16;
    
    uint32_t shift_bits = 32 - prefix;
    uint32_t host_ipv4 = ipv4 >> shift_bits;  //网络段 1.1.0.0是不变的
    host_ipv4++;  //先算出下一个网段 0x102
    //1.1.0.0网段最后一个地址是0x1020000 -1
    uint32_t end_ipv4 = (host_ipv4<<shift_bits)-1; // 101ffff
    printf("start ip: %x end ip: %x\n",ipv4,end_ipv4);
    end_ipv4 = htonl(end_ipv4); // ffff0101
    char ip_str[20]={0};
    inet_ntop(AF_INET, &end_ipv4, ip_str, INET_ADDRSTRLEN); // 1.1.255.255
    printf("end ip: %s \n",ip_str);

    return 0;
}

$ gcc -o ip_calculator ip_calculator.c 
$ ./ip_calculator
start ip: 1010000 end ip: 101ffff
end ip: 1.1.255.255 

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

#define uint8_t unsigned char
#define uint32_t unsigned int

int main(int argc,char **argv)
{
    uint8_t ipv6[16]={0};
    char IPdotdec[46]="1:1:1:eeff::"; 
    inet_pton(AF_INET6, IPdotdec, (void *)&ipv6);
    printf("十六进制 : %s \n",IPdotdec);

    uint8_t  prefix = 54;
    uint32_t shift_bits = 128 - prefix;
    uint32_t total_shift_byte,remain_shift_bits,j;
    uint8_t host_ipv6 = 0, host2_ipv6 = 0,host_max = 0xFF;
    uint8_t host_ipv6_str = 0;
    uint8_t reach_host_max = 0;
    total_shift_byte = shift_bits/8;  //6   
    remain_shift_bits = shift_bits%8; //6 


    for(j = 0; j < total_shift_byte+1; j++)
    {
        if(j < total_shift_byte){
        //e.g. prefix_len = 48. addr[0] ~ [5] is fixed,addr[6]~[15] is variable init to 0
          ipv6[15-j] = 0xff;
        }
        else if(remain_shift_bits)
        {
          //just parts of this byte shitf to 0
          host_ipv6 = ipv6[15-j]>> remain_shift_bits;   
          host_ipv6++;
          host_ipv6 = (host_ipv6<<remain_shift_bits)-1;   
          ipv6[15-j]= host_ipv6 ;
          break; // find the fix addr[15-j],case 1:just 1 byte affected
        }
        printf("ipv6[%u]:0x%x %x remain_shitf %u\n",15-j,ipv6[15-j],host_ipv6,remain_shift_bits);
    }
    char ipv6_str1[46]={0};       
    inet_ntop(AF_INET6,ipv6, ipv6_str1,INET6_ADDRSTRLEN);
    printf("ip %s, shift addr[%u]\n",  ipv6_str1, 15-j);

    return 0;
}

$ ./ipv6_calculator
十六进制 : 1:1:1:eeff:: 
ipv6[15]:0xff 0 remain_shitf 2
ipv6[14]:0xff 0 remain_shitf 2
ipv6[13]:0xff 0 remain_shitf 2
ipv6[12]:0xff 0 remain_shitf 2
ipv6[11]:0xff 0 remain_shitf 2
ipv6[10]:0xff 0 remain_shitf 2
ipv6[9]:0xff 0 remain_shitf 2
ipv6[8]:0xff 0 remain_shitf 2
ipv6[7]:0xff 0 remain_shitf 2
ip 1:1:1:efff:ffff:ffff:ffff:ffff, shift addr[6]