From: elchino@cnn.cn   
      
   I'm having some troubles figuring out the CIDR "prefix length"   
   of a IPv6 addresses range.   
      
   Specifically, in some code that reads MaxMind's GeoIPv6.dat records,   
   the low and high IP-numbers of IPv6-blocks are given like:   
     IP-num low:  76:96:42:219::   
     IP-num high: 76:96:42:219:ffff:ffff:ffff:ffff   
     (belonging to 'United States')   
      
   I need to figure out how to print this using CIDR notation.   
   E.g. if I have the 2 above addresses in   
     struct in6_addr a, b;   
      
   I've cooked up this function after much head-scratching:   
      
     static int network_len128 (const struct in6_addr *a, const struct in6_addr   
   *b)   
     {   
       int  i, j, bits = 0;   
       BYTE v;   
      
       for (i = 15; i >= 0; i--)   
       {   
         v = (a->s6_bytes[i] ^ b->s6_bytes[i]);   
         for (j = 0; j < 8; j++, bits++)   
             if ((v & (1 << j)) == 0)   
                goto quit;   
       }   
     quit:   
       return (bits);   
     }   
      
   Figuring I had to go backwards from LSB to MSB and stop counting   
   when 2 bits are equal.   
      
   The above code (+inet_ntop(&a..)) will print "76:96:42:219::/64".   
   Does this make sense for you experts?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)