The third layer of the OSI model. This layer is used to label the network 
addresses. There are different network layer executions for different purposes. For instance, internet protocol (IP) is used to define network addresses. This makes route selection determinable in a systematic way through the comparison of the source network address and the destination network address and through application of the subnet mask. The internet protocol address has inherent advantages. It makes the setting up and connection of networks an easy task. The internet for example, makes use of the internet protocol (IP) to connect millions of different networks from different parts of the word. A network protocol uses a 32 bit IP address to identify a host. IP addresses consist of four dots separated decimal numbers from 0 to 255.
Again, the network is used in the definition of the logical layer network. For this purpose, the network can then be used by routers to determine how to send data packets. In certain instances, the network layer is often separated into subnets. This makes network management that much easier plus it also controls the flow of data packets. Routers then make use of the networks or the subnets of the IP addressing for purposes of routing the traffic of data between different networks. Consequently, each router is specifically configured for the networks and the subnets that may be linked into it.
A routing protocol is used by one router to communicate with another router. Examples of routing protocols include the Routing Information Protocol (RIP) and the Open version of Shortest Path First (OSPF). The importance of the routing protocol is that it makes routers aware of the existence and presence of other networks thereby helping them to calculate the most effective way of reaching each of the networks. Therefore, routing decisions undertaken by routers and networked systems take place in the network layer. It is also in the network layer where fragmentation of data packets occurs. Fragmentation is necessary so that packets may fit in with the data link protocol of the destination. Due to this, it is in the network layer that much of the internet, design and configuration activities take place. Thus, this network layer provides a unique type of addressing system; end to end. The purpose of this system is the routing of a data packet across several data link layers for instance, Token Ring and/or Ethernet.
