This paper describes an access control mechanism that enforces at the network level an access control decision that is taken at the application level. The mechanism is based on the pre-computation of encrypted counters called tickets. An access enforcement device verifies the existence of a valid ticket in each packet that is subject to access control and kills unauthorized packets. Tickets are not computed as a function of the user data. Due to the timing constraints of shared media LANs the presence of a valid ticket in a packet proves that the operation implied by the user data has been authorized. The access control mechanism is elaborated for Internet protocols over Ethernet and we discuss its properties for internetworking and multicasting.