Hello Maodo
According to RFC2474 it speaks about the DS field:
Implementors should note that the DSCP field is six bits wide. DS-
compliant nodes MUST select PHBs by matching against the entire 6-bit
DSCP field, e.g., by treating the value of the field as a table index
which is used to select a particular packet handling mechanism which
has been implemented in that device. The value of the CU field MUST
be ignored by PHB selection.
It says that the entire 6-bit field must be used. The bits in question are bits 0 to 5 (a total of six bits). Bits 6 and 7, also known as the CU bits, are currently unused. Could this be the “6th bit” that you mention should be ignored?
Even in RFC791 (IP Header Structure) which precedes the above RFC, the then called Type of Service or ToS field used all six bits as can be seen from the link.
I hope this has been helpful!
Laz