Idle bandwidth sharing is a promising concept in wireless networking, especially in vehicular ad-hoc network (VANET), where the peers in VANET usually have different networking bandwidth. It helps vehicle with extra Internet access bandwidth obtained from idle bandwidth enabled peers when its bandwidth is not enough for large bandwidth services such as telematics, streaming services and location-based services. However, there are several issues including authority, bandwidth competition, low idle bandwidth utilization and heavy message overhead problems in traditional bandwidth sharing schemes. In this paper, a Hierarchical M-hops Idle Bandwidth Sharing Approach (HMIBSA) is proposed to reduce the messaging overhead comparing to previous work (Authorized Idle Bandwidth Sharing Approach, AIBSA) which is a centralized planning approach. Two ideas are introduced in HMIBSA including M-hops grouping and hierarchical two-level planning. The former limits the message broadcasting range, and the latter has the advantages of distributed computing while keep the global view advantage in AIBSA. The idle bandwidth planning algorithm generates multiple authorized paths for creating the paths between bandwidth demanders and owners by considering overall idle bandwidth demands and supplies, global idle bandwidth utilization efficiency, and authorization policies. HMIBSA solves the problems in traditional bandwidth sharing schemes, and the simulation results show that the HMIBSA performs well in both the global utilization of the idle bandwidth and the message overhead.