Network layer
Network Layer
The network layer is responsible for the source-to destination delivery of a packet, possibly across multiple networks (links). Whereas the data link layer oversees the delivery of the packet between two system on the same network (link), the network layer ensures that each packet gets from its point if origin to its final destination.
If two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached two different networks (link) with connecting devices between the networks (link), there is often a need for the network layer to accomplish source-to-destination delivery. Other responsibilities of the network layer include the following:
Ø Logical addressing. The physical addressing implemented by the data link layer handles the addressing problem locally. If a packet passes the network boundary, we need another addressing system distinguish the source and destination system. The network layer adds a header to the packet coming from the upper layer that, among other thing, includes the logical addresses of the sender and receiver.
Ø Routing. When independent networks or links are connected together to create internetworks (network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the function of the network layer is to provides this mechanism.
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