A method for transmitting a first field and one or more second fields is described. A number of devices in a group of multiple devices to which a first OFDMA data unit is to be transmitted is selected. A block allocation that indicates respective integer numbers of different tone blocks of a WLAN communication channel to be assigned to each device in the group of multiple devices is selected. A first field is encoded to indicate both the selected number of devices in the group and the selected block allocation. One or more second fields are encoded to indicate a respective device identifier for each device in the group of multiple devices. The first field and the one or more second fields are transmitted to each device in the group of multiple devices.