The strict boundary of the five layers in the TCP/IP network model provides the information encapsulation that enables the standardizing of network communications and makes the implementation of networks convenient in terms of abstract layers. However, the encapsulation results in some side effects, including compromise of QoS, latency, extra overload, etc. Therefore, to mitigate the side effect of the encapsulation between the abstract layers in the TCP/IP model, a number of cross-layer designs have been proposed. Cross-layer designs allow information sharing among all of the five layers in order to improve the wireless network functionality, including security, QoS, and mobility. In this article, we classify cross-layer designs by two ways. On the one hand, by how to share information among the five layers, cross-layer designs can be classified into two categories: non-manager method and manager method. On the other hand, by the organization of the network, cross-layer designs can be classified into two categories: centralized method and distributed method. Furthermore, we summarize the challenges of the cross-layer designs, including coexistence, signaling, the lack of a universal cross-layer design, and the destruction of the layered architecture.