The third-lowest layer of the OSI Reference Model is the network layer. If the data link layer defines the boundaries of what is considered a network, the network layer defines how internetworks (interconnected networks) function. The network layer is the lowest one in the OSI model that is concerned with actually getting data from one computer to another even if it is on a remote network; in contrast, the data link layer only deals with devices that are local to each other.
Some of the specific jobs normally performed by the network layer include the following :
Logical Addressing - Every device that communicates over a network has a logical address associated with it, which identifies the device regardless of its particular location. This is sometimes called a layer 3 address. For example, on the Internet, the Internet Protocol (IP) is the network layer protocol and every machine has an IP address. Logical addresses are independent of particular hardware and must be unique across an entire internetwork.
|NOTE : Addressing is done at the data link layer as well, but those addresses refer to local physical devices.|
Routing - The defining function of the network layer is routing—moving data across a series of interconnected networks. It is the job of the devices and software routines that function at the network layer to handle incoming packets from various sources, determine their final destination, and then figure out where they need to be sent to get them where they are supposed to go.
Datagram Encapsulation - The network layer normally encapsulates messages received from higher layers by placing them into datagrams (also called packets) with a network layer header.
Fragmentation and Reassembly - The network layer must send messages down to the data link layer for transmission. Some data link layer technologies limit the length of any message that can be sent. If the packet that the network layer wants to send is too large, the network layer must split the packet up (fragment it), send each piece to the data link layer, and then have the pieces reassembled once they arrive at the network layer on the destination machine.
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