The paper addresses a bandwidth selection problem while channel bonding scheme is used in IEEE 802.11ac wireless local area networks (WLANs) such that a channel with appropriate bandwidth can be utilized by STAs to transmit without collision. The goal of IEEE 802.11ac is to provide very high throughput (VHT) WLANs over 5 GHz band. Among the new technologies specified in IEEE 802.11ac, channel bonding is regarded as a quite useful scheme to accommodate growing traffic demands in WLANs. However, to effectively exploit the efficiency of channel bonding scheme, a well-designed MAC protocol is of crucial importance. As a result, the transmission in a wider bandwidth is analyzed first. Based on the results, two problems, Carrier Sensing Decreasing Problem (CSDP) and Outside Warning Range Problem (OWRP), are conducted in this paper. Moreover, the paper analyzes the relationships among the transmission range, carrier-sensing range, and interference range in case that channel bonding is adopted. Consequently, a dynamic bandwidth selection (DBS) MAC protocol for IEEE 802.11ac WLANs is proposed to avoid collisions. DBS can not only prevent CSDP and OWRP problems but also can increase network performance. Simulation results show that DBS outperforms against other protocols in packet successful rate and network throughput.