Alteration of data packet check data

A node of a computer network receives an input data packet comprising communication data and check data. The communication data is altered, for instance by the insertion or removal of a VLAN tag, and new check data is calculated. An output data packet comprising the altered communication data and new check data is intentionally corrupted if the unaltered communication data is found to contain errors when checked with the check data. The chance of undetected corruption of the communication data within the node is minimised by providing memory means 20 from which the communication data is read in its unaltered or altered form and supplied to both the checking means 26 and new check data generating means 28 on the same bus 24.