This article is from the Ethernet FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.
When a station sends a frame, it appends a Cyclical Redundancy
Check to the end of the frame. This CRC has been generated from an
algorithm and is based on the data in the frame. If the frame is
altered between the source and destination, the receiving station
will recognize that the CRC does not match the actual contents of
All frames should end on an 8-bit boundary, but problems on the
network could cause the number of bits to deviate from the multiple
Both CRC errors and alignment errors are grouped together as the
single CRC/Alignment error counter.