Abstract: A method and apparatus for generating an error detection code, such as a Cyclic Redundancy Checksum (CRC), for a modified binary data block. The modified data block, such as a VLAN frame, is derived from an original binary data block, such as an ethernet frame, having a first error detection code associated therewith. In one embodiment, the method requires modifying the original data block utilizing first data, in the form of VLAN header information, to generate the VLAN frame, whereafter a second error detection code is calculated exclusively for the VLAN header information. More specifically, where the original data block is modified by the insertion of the VLAN header information into the original data block, a CRC is calculated using the VLAN header information shifted to a position corresponding to its position within the modified data block.