In this paper, a unified analytical model is established to analyze the throughput performance of M-group heterogeneous IEEE 802.11 DCF networks with a distinct input rate, number of nodes and initial backoff window size in each group. Steady-state points under unsaturated, partially-saturated and fully-saturated scenarios are characterized, and the maximum throughput is derived. Explicit expressions of the optimal initial backoff window sizes to achieve the maximum throughput are also obtained, which shed important light on the practical network design and configuration of IEEE 802.11 DCF networks.