Protocols at the network layer are also responsible for route selection, which refers to determining the best path for the data to take throughout the network. In contrast to the data-link layer, which uses MAC addresses to communicate on the LAN, network protocols use software configured addresses and special routing protocols to communicate on the network. The term packet is used to describe the logical grouping of data at the data-link layer.
Transport Layer (Layer 4)
The basic function of the transport layer is to provide mechanisms to transport data between network devices. Primarily it does this in three ways:
Error checking Protocols at the transport layer ensure that data is sent or received correctly.
Service addressing Protocols such as TCP/IP support many network services. The transport layer makes sure that data is passed to the right service at the upper layers of the OSI model.
Segmentation To traverse the network, blocks of data need to be broken down into packets that are of a manageable size for the lower layers to handle. This process, called segmentation, is the responsibility of the transport layer.