A method for registration of multiple entities belonging to a specific optical networks unit (ONU). In one embodiment, the multiple entity registration method comprises checking by an optical line terminal (OLT) if a registration request message (400) received from the specific ONU belongs to a certain grant (402), and based on the check result, registering an entity as either a first (408) or as an additional entity (404) of the specific ONU. In another embodiment, the method comprises checking by an OLT of a reserved value of a flags field (502) inside a registration request message (500), and based on the check result, registering an entity as either a first (508) or as an additional entity (504) of the specific ONU. The knowledge by an OLT that multiple entities belong to a specific ONU is used for grant optimization and packet data flow optimization.