Which QoS concept uses 64 service classes at layer-3 by marking packets?

• A. Best Efforts Service
• B. Differentiated Services
• C. Class of Service
• D. WMM

Answer: (B)

Differentiated Services is about labeling packets in the IP header so routers can treat them differently as they pass through the network. This labeling is done using the Differentiated Services Code Point (DSCP), a 6-bit field in the IP header. Because there are 6 bits, you can encode up to 2^6, or 64, distinct service classes. Edge devices or hosts mark packets with a DSCP value, and routers along the path use that value to apply a corresponding per-hop behavior (such as higher priority, more bandwidth, or low delay). This is a layer-3 mechanism, focusing on how packets are handled inside IP networks rather than on the Ethernet switch’s behavior.

So the idea of assigning up to 64 service classes by marking packets aligns with Differentiated Services. In contrast, Best Efforts is the default with no special markings, Class of Service typically refers to layer-2 802.1Q CoS concepts, and WMM pertains to wireless QoS with a small number of categories, not 64 at the IP layer.