'API Version 2014-10-01 defines the following filters for DescribeSubnets:
* availabilityZone
* available-ip-address-count
* cidrBlock
* defaultForAz
* state
* subnet-id
* tag:key=value
* tag-key
* tag-value
* vpc-id
Taken from: http://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeSubnets.html'

def get_filter_value(self, filter_name):

    if (filter_name in (u'cidr', u'cidrBlock', u'cidr-block')):
        return self.cidr_block
    elif (filter_name in (u'vpc-id', u'vpcId')):
        return self.vpc_id
    elif (filter_name == u'subnet-id'):
        return self.id
    elif (filter_name in (u'availabilityZone', u'availability-zone')):
        return self.availability_zone
    elif (filter_name in (u'defaultForAz', u'default-for-az')):
        return self.default_for_az
    filter_value = super(Subnet, self).get_filter_value(filter_name)
    if (filter_value is None):
        self.ec2_backend.raise_not_implemented_error(u"The    filter    '{0}'    for    DescribeSubnets".format(filter_name))
    return filter_value
