I’ve posted this before, buyt now that I have better source code formatting, I’ll repost. This converts a mac address to a link only IPv6 address.

mac2ipv6addr.c

#include 
#include 
#include 
#include 

int main(int argc, char** argv){

  int addrlen = strlen("0000:0000:0000:0000:0000:0000:0000:");
  char* out = malloc(addrlen);
  char * outorig = out;

  memset(out, 0, addrlen);

  char* addr =    "00:0c:29:20:4e:e3";

  if (argc > 1){
    addr = argv[1];
  }else{
    fprintf(stderr,"usage %s macaddr\n",argv[0]);
    exit(-1);
  }

  int len = strlen(addr);

  if (len > 18){
    printf ("String too long\n");
    exit(-1);
  }

  /*We know we have the right length.  Main processing follows */

  int i ;
  int col_count = 0;
  unsigned char current = 0;
  for (i = 0; i < len; ++i){
    char c = addr[i];

    if (0 == c){
      break;
    }else if (':' == c){
      switch( col_count ){
      case 0:{
        sprintf(out,"fe80::");
        out += strlen(out);

        /*Toggle the '2' bit*/
        unsigned short c2 = ( current | 0x02 );
        if (c2 == current){
          c2 = current & 0xcf;
        }

        sprintf(out,"%02x",c2);
        out += strlen(out);
      }
        break;
 case 2:{
        /*The magic number goes halfway through the mac address*/
        sprintf(out,"%02xff:fe",current);
        out += strlen(out);
      }
        break;
      default:
        sprintf(out,"%02x",current);
        out += strlen(out);

        if (col_count % 2){
          sprintf(out,":");
          out += strlen(out);
        }
      }
      ++col_count;
      current = 0;
    }else if ((c >= 'a') && (c <= 'f')){
      current *=16;
      current += ( 10 + c - 'a');
    }else if ((c >= 'A') && (c <= 'F')){
      current *=16;
      current += ( 10 + c - 'a');
    }else if ((c >= '0') && (c <= '9')){
      current *=16;
      current += ( c - '0');
    }
  }

  sprintf(out,"%x",current);
  out += strlen(out);
  printf(outorig);
  return 0;

}

Ignore that last line. Not sure why the formatting code is closing my open tags insde a pre tag...