Which bits can network administrators use to define custom DiffServ packet PHBs?

The correct choice indicates that network administrators can use bits 3 to 5 to define custom Differentiated Services (DiffServ) Per Hop Behaviors (PHBs). In the DiffServ architecture, the Differentiated Services Code Point (DSCP) is a 6-bit field in the IP header that is used to classify packets for different levels of service in the network. While the first three bits of the DSCP are used to define standard PHBs, bits 3 to 5 are reserved for defining custom PHBs.

Using these bits allows administrators to create specific service levels tailored to their network's requirements, enabling them to prioritize or manage bandwidth for certain types of traffic more effectively. This flexibility in packet treatment plays a crucial role in modern network management, especially for applications that require stringent performance parameters, such as voice over IP or video conferencing.

Other bits in the DSCP field serve different functions and are either reserved for other use cases or predefined by the standards, which is why focusing on bits 3 to 5 is essential for defining custom behaviors.