Question : What are the responsibilities of the network layer in the TCP/IP protocol?
Under TCP/IP, the network layer is called the IP layer. The network layer is responsible for moving data between machines across the boundaries of physical networks. For example, the IP layer is responsible for moving data off the office ethernet, across the company network backbone , and out onto the Internet.
The key capability added by the network layer is routing. Routing is the process that directs information among a choice of paths based on the intended destination. In the case of TCP/IP, routing is based on IP addresses.
The network layer ensures that data intended for a machine with a specified IP address arrives at its destination.
The routing problem is extremely important, and is usually handled by computers specially designed to make routing decisions.
These special-purpose computers are called routers.