import six
import re


def is_valid_mac(address):
    """Verify the format of a MAC address.

    Check if a MAC address is valid and contains six octets. Accepts
    colon-separated format only.

    :param address: MAC address to be validated.
    :returns: True if valid. False if not.

    """
    m = "[0-9a-f]{2}(:[0-9a-f]{2}){5}$"
    if isinstance(address, six.string_types) and re.match(m, address.lower()):
        return True
    return False