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):
        """
        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
        """
        if filter_name in ('cidr', 'cidrBlock', 'cidr-block'):
            return self.cidr_block
        elif filter_name in ('vpc-id', 'vpcId'):
            return self.vpc_id
        elif filter_name == 'subnet-id':
            return self.id
        elif filter_name in ('availabilityZone', 'availability-zone'):
            return self.availability_zone
        elif filter_name in ('defaultForAz', 'default-for-az'):
            return self.default_for_az
        else:
            return super(Subnet, self).get_filter_value(
                filter_name, 'DescribeSubnets')