Final answer:
The massive exchange network to which mature Harappan sites contributed is likely an extensive trade system involving the civilization's urban centers and connections with other civilizations, such as Mesopotamia.
Step-by-step explanation:
The network to which mature Harappan sites contributed has been proposed to be a large and complex exchange system, likely involving a combination of independent city-states and possibly regional capitals. The Indus Valley Civilization, often referred to as the Harappan civilization due to Harappa being one of its major cities, developed a sophisticated and standardized urban planning that facilitated trade. This network extended beyond the Indus Valley, with Merchants from Sumer traveling to the region to exchange luxury goods such as lapis lazuli, highlighting a two-way trade connection. In this way, the Indus Valley not only contributed to but also integrated with a more massive exchange network that included various civilizations such as Mesopotamia.