new ec2.Subnet(stack, 'MySubnet', {
  vpcId: vpc.vpcId,
  availabilityZone: 'us-east-1a',
  ipamAllocation: {
    ipamPoolId: 'ipam-pool-123',
    netmaskLength: 24
  }
});

export interface SubnetProps {
  // ... existing properties ...

  /**
   * IPAM allocation configuration
   * If specified, cidrBlock must not be set
   * @default - no IPAM allocation
   */
  readonly ipamAllocation?: {
    /**
     * The ID of the IPAM pool to allocate the CIDR from
     */
    readonly ipamPoolId: string;

    /**
     * The netmask length for the CIDR
     * Must be a value between 16 and 28
     */
    readonly netmaskLength: number;
  };
}

export class Subnet extends Resource implements ISubnet {
  constructor(scope: Construct, id: string, props: SubnetProps) {
    super(scope, id);

    if (props.ipamAllocation && props.cidrBlock) {
      throw new Error('Cannot specify both ipamAllocation and cidrBlock');
    }

    const subnet = new CfnSubnet(this, 'Resource', {
      vpcId: props.vpcId,
      availabilityZone: props.availabilityZone,
      // ... other properties ...
      
      // Add IPAM properties when specified
      ...(props.ipamAllocation && {
        ipv4IpamPoolId: props.ipamAllocation.ipamPoolId,
        ipv4NetmaskLength: props.ipamAllocation.netmaskLength
      })
    });

    // ... rest of the implementation ...
  }
}

test('subnet with ipam allocation', () => {
  const stack = new Stack();
  new Subnet(stack, 'Subnet', {
    vpcId: 'vpc-123',
    availabilityZone: 'us-east-1a',
    ipamAllocation: {
      ipamPoolId: 'ipam-pool-123',
      netmaskLength: 24
    }
  });

  Template.fromStack(stack).hasResourceProperties('AWS::EC2::Subnet', {
    VpcId: 'vpc-123',
    AvailabilityZone: 'us-east-1a',
    Ipv4IpamPoolId: 'ipam-pool-123',
    Ipv4NetmaskLength: 24
  });
});

test('cannot specify both cidrBlock and ipamAllocation', () => {
  const stack = new Stack();
  expect(() => {
    new Subnet(stack, 'Subnet', {
      vpcId: 'vpc-123',
      availabilityZone: 'us-east-1a',
      cidrBlock: '10.0.0.0/24',
      ipamAllocation: {
        ipamPoolId: 'ipam-pool-123',
        netmaskLength: 24
      }
    });
  }).toThrow(/Cannot specify both ipamAllocation and cidrBlock/);
});

export class Vpc extends Resource implements IVpc {
  private allocateSubnet(props: AllocateSubnetProps): Subnet {
    // ... existing allocation logic ...
    
    // If VPC uses IPAM, prefer VPC's IPAM settings unless subnet explicitly specifies its own
    const ipamAllocation = props.ipamAllocation ?? (this.ipamPool && {
      ipamPoolId: this.ipamPool.poolId,
      netmaskLength: this.defaultSubnetMask
    });

    return new Subnet(this, subnetId, {
      // ... other properties ...
      ipamAllocation,
    });
  }
}