def in6_get_common_plen(a, b):
    """
    Return common prefix length of IPv6 addresses a and b.
    """
    def matching_bits(byte1, byte2):
        for i in range(8):
            cur_mask = 0x80 >> i
            if (byte1 & cur_mask) != (byte2 & cur_mask):
                return i
        return 8

    tmpA = inet_pton(socket.AF_INET6, a)
    tmpB = inet_pton(socket.AF_INET6, b)
    for i in range(16):
        mbits = matching_bits(orb(tmpA[i]), orb(tmpB[i]))
        if mbits != 8:
            return 8 * i + mbits
    return 128