The network layer is completing the job of translating the network addresses into lower level language and finding a route how data could be delivered.
Network equipment is having two types of addresses that help simplify delivery. There is a Network address (Network layer address, logical address, virtual address) and a physical address. If we compare the Network address with the persons living address we would find many similarities as shown in Figure 1.
Figure 1. Network Layer in OSI Interworking System
Just like location address the area of the deliverer location is being narrowed down until there is only one node left. There are many Network layer protocols having different formats. When the network layer receives the segments of data from Transport layer it converts them to the packets by adding addressing information to them. After that Network layer finds a route for data to be delivered to the receiver. Route may differ because it is being determined based on network overload, Quality of Service, cost of alternative routes, and delivery priorities. Routing is a task performed by network layer, which makes routers to belong to the Network layer.
In the example of user using the web browser for the web-page access the Internet Protocol (IP) of the Network layer is being used. It defines the source and the receiver in the network.
Additional function of the Network layer is fragmentation. It is similar to segmentation process in Transport layer and is being performed only if segmentation was not enough or the Transport layer protocol did not perform segmentation in the first place.
