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The Network layer is responsible for routing through an internetwork and for network addressing. This means that the Network layer is responsible for transporting traffic between devices that are not locally attached. Routers, or other layer-3 devices, are specified at the Network layer and provide the routing services in an internetwork.
When a packet is received on a router interface, the destination IP address is checked. If the packet is not destined for the router, then the router will look up the destination network address in the routing table. Once an exit interface is chosen, the packet will be sent to the interface to be framed and sent out on the local network. If the entry for the destination network is not found in the routing table, the router drops the packet. Two types of packets are used at the network layer: data and route updates.
Data packets Are used to transport user data through the internetwork, and protocols used to support data traffic are called routed protocols. Examples of routed protocols are IP and IPX.