for ip in addresses[1:]:

        if ip._ip == last._ip + 1:

            last = ip

        else:

            break

    return (first, last)

def _get_prefix_length(number1, number2, bits):

    """Get the number of leading bits that are same for two numbers.

    Args:

        number1: an integer.

        number2: another integer.

        bits: the maximum number of bits to compare.

    Returns:

        The number of leading bits that are the same for two numbers.

    """

    for i in range(bits):

        if number1 >> i == number2 >> i:

            return bits - i

    return 0

def _count_righthand_zero_bits(number, bits):

    """Count the number of zero bits on the right hand side.

    Args:

        number: an integer.

        bits: maximum number of bits to count.

    Returns:

        The number of zero bits on the right hand side of the number.

    """

    if number == 0:

        return bits

    for i in range(bits):

        if (number >> i) % 2:

            return i

def summarize_address_range(first, last):

    """Summarize a network range given the first and last IP addresses.

    Example:

    Args:

        first: the first IPv4Address or IPv6Address in the range.

        last: the last IPv4Address or IPv6Address in the range.

    Returns:

        The address range collapsed to a list of IPv4Network's or

        IPv6Network's.

    Raise:

        TypeError:

            If the first and last objects are not IP addresses.

            If the first and last objects are not the same version.

        ValueError:

            If the last object is not greater than the first.

            If the version is not 4 or 6.

    """

    if not (isinstance(first, _BaseIP) and isinstance(last, _BaseIP)):

        raise TypeError('first and last must be IP addresses, not networks')

    if first.version != last.version:

        raise TypeError("%s and %s are not of the same version" % (

                str(first), str(last)))

    if first > last:

        raise ValueError('last IP address must be greater than first')

    networks = []

    if first.version == 4:

        ip = IPv4Network

    elif first.version == 6:

        ip = IPv6Network

    else:

        raise ValueError('unknown IP version')

    ip_bits = first._max_prefixlen

    first_int = first._ip

    last_int = last._ip

    while first_int <= last_int:

        nbits = _count_righthand_zero_bits(first_int, ip_bits)

        current = None

        while nbits >= 0:

            addend = 2 ** nbits - 1

            current = first_int + addend

            nbits -= 1

            if current <= last_int:

                break

        prefix = _get_prefix_length(first_int, current, ip_bits)