In VLSI systems, Rent's rule characterizes the locality of interconnect between different subsystems, and allows an efficient layout of the circuit on a chip. With rising complexities of both hardware and software, Systems-on-Chip are converging to multiprocessor architectures connected by a Network-on-Chip. Here, packets are routed instead of wires, and threads of a parallel program are distributed among processors. Still, Rent's rule remains applicable, as it can now be used to describe the locality of network traffic. In this paper, we analyze network traffic on an on-chip network and observe the power-law relation between the size of clusters of network nodes and their external bandwidths. We then use the same techniques to study the time-varying behavior of the application, and derive the implications for future on-chip networks.