def _find_vlan(mac, domain_interfaces):
    """
    Given a mac address and a collection of domains and their network
    interfaces, find the domain that is assigned the interface with the
    desired mac address.

    Parameters
    ----------
    mac : str
        The MAC address.
    domain_interfaces : dict
        The list of domain interfaces.

    Returns
    -------
        The domain
    """
    for d, i in domain_interfaces.items():
        if i.get(mac):
            return d

    return None