Page "Padding (cryptography)" Paragraph 10

A single set (' 1 ') bit is added to the message and then as many reset (' 0 ') bits as required ( possibly none ) are added.

The number of reset (' 0 ') bits added will depend on the block boundary to which the message needs to be extended.

In bit terms this is " 1000 ... 0000 ", in hex byte terms this is " 80 00 ... 00 00 ".