A random number generating apparatus for an interface unit of a Carrier Sense with Multiple Access and Collision Detect (CSMA/CD) Ethernet data network. The interface unit includes a transmit backoff unit for implementing a backoff algorithm in response to a network collision signal and a random number. The apparatus comprises a dual mode random number generator and a multiplexer for switching the random number generator between modes in accordance with the serial address bits of a data packet being processed by the interface unit. The random number generator includes a 25 stage linear feedback shift register. The multiplexer has two signal inputs connected to outputs of the 18th and 22nd stages of the shift register respectively, a switch input connected to receive the serial address bits and an output connected in circuit to an input of the shift register.