ToS bits 6 and 7 are Reserved

The Type of Service (ToS) byte was defined in RFC 791 way back in 1981 when IPv4 was also defined. The ToS byte as part of the IPv4 header and looks like this:

The Type of Service section of the ToS Byte refers to bits 3-7. Don't confuse the ToS byte with the Type of Service section of the ToS byte.

Bits 3 to 5 were defined, but bits 6 and 7 were "reserved for future use." In the context of ToS and IP Precedence as originally defined, these bits didn't end up being used in this framework.

The reservation of these two bits should not be confused with the reservation of class selector values 6 and 7 being reserved for Internetwork Control and Network Control in the context of Class Selector PHB.

Links

https://networklessons.com/quality-of-service/ip-precedence-dscp-values#IP_Precedence