'Split into v4, v6 stateless and v6 stateful subnets'

def _classify_subnets(self, context, subnets):

    (v4, v6_stateful, v6_stateless) = ([], [], [])
    for subnet in subnets:
        if (subnet['ip_version'] == 4):
            v4.append(subnet)
        elif (not ipv6_utils.is_auto_address_subnet(subnet)):
            v6_stateful.append(subnet)
        else:
            v6_stateless.append(subnet)
    return (v4, v6_stateful, v6_stateless)
