A method of configuring an optical network terminal, ONT, of a wavelength division multiplexed passive optical network, WDM PON. Sequentially setting a transmission wavelength of a tunable optical filter at the ONT to one or more wavelengths of a pre-selected plurality of transmission wavelengths until a wavelength is identified for which no downstream optical signal from an optical line terminal of the WDM PON is detected. Determining whether a transmitter operable at said identified transmission wavelength is present at the OLT and is available to be assigned to communicate with the ONT. If a transmitter is determined to be present and available, maintaining the transmission wavelength of the tunable optical filter at transmission wavelength and assigning the ONT to the OLT as a subscriber at said identified transmission wavelength. If a transmitter is determined to be one of not present and not available, recommencing the method.