```C
#include <iostream>
#include <string>
#include <string.h>
#include <stdio.h>

int main()
{
    char* ptr = NULL;
    char ip_addr[]="1:2:3:4:ffff:ffff:1ff:fff6";
    int size = 0;
    char sIpv6AddrEnd[64]  = {0,};

    unsigned int numberOfLease = 10;

    unsigned long long ipv6AddrStart = 0;
    unsigned long long ipv6AddrEnd = 0;  // 64bit
    unsigned int ipv6AddrHextet;

    ptr = strtok(ip_addr, ":");

    /* Convert IPv6 address to unsigned long long */
    for (int nCount = 0 ; nCount < 7 ; nCount++)
    {
        ptr = strtok (NULL, ":");
        if (nCount >=3)
        {
            ipv6AddrHextet = strtoul (ptr, NULL, 16);
            ipv6AddrStart = (ipv6AddrStart << 16) + ipv6AddrHextet;
        }
    }

    ipv6AddrEnd = ipv6AddrStart + numberOfLease;
    /* Check over flow */
    if (ipv6AddrEnd < ipv6AddrStart)
    {
        std::cout << "\nError\n";
    }
    else
    {
        ipv6AddrHextet = ipv6AddrEnd >> 48;
        size += snprintf(sIpv6AddrEnd ,16, "%x", ipv6AddrHextet);
        ipv6AddrHextet = (ipv6AddrEnd << 16) >> 48;
        size += snprintf(sIpv6AddrEnd + size ,16, ":%x",ipv6AddrHextet);
        ipv6AddrHextet = (ipv6AddrEnd << 32) >> 48;
        size += snprintf(sIpv6AddrEnd + size ,16, ":%x", ipv6AddrHextet);
        ipv6AddrHextet = (ipv6AddrEnd << 48) >> 48;
        size += snprintf(sIpv6AddrEnd + size ,16, ":%x", ipv6AddrHextet);
    }

    std::cout << " sIpv6AddrEnd: " << sIpv6AddrEnd << "\n";
    return 0;
}
```