def in6_ptoc(addr):
    """
    Converts an IPv6 address in printable representation to RFC
    1924 Compact Representation ;-)
    Returns None on error.
    """
    try:
        d = struct.unpack("!IIII", inet_pton(socket.AF_INET6, addr))
    except Exception:
        return None
    res = 0
    m = [2**96, 2**64, 2**32, 1]
    for i in range(4):
        res += d[i] * m[i]
    rem = res
    res = []
    while rem:
        res.append(_rfc1924map[rem % 85])
        rem = rem // 85
    res.reverse()
    return "".join(res)