Due to a high cost of 3D IC process technology, the semiconductor industry is targeting 2.5D ICs with interposer as a fast and low-cost alternative to integrate dissimilar technologies. In this paper, we propose an independent network-on-chip die, called Network-on-Die (NoD), for 2.5D ICs that operates as a communication backbone for heterogeneous many-core systems on interposer. NoD is responsible for routing packets from a source router to a destination router, and the connections between routers and cores pass through the interposer. This technique eliminates the complexity of the routing algorithms in heterogeneous systems by turning the irregular form of NoC in 2.5D ICs into a regular/optimized one in NoD. The performance evaluation is verified through RTL simulations for a heterogeneous many-core system of varying die sizes and with asymmetric shapes. We provide the theoretical justification for our simulation results.