At the conceptual level, we can think of the global internet as a network in the form of a black box that connects millions together /billions of computers worldwide.

We will only address how a message from the application layer on one computer reaches the application layer on another computer. At this conceptual level, we can think about the communication between A and B.

However, the Internet is not a single network, but a union of many networks (or connected together through connecting devices. In other words, the Internet is an Internetwork, Internetwork, (a combination of LANs and WANs. To better understand the role of the network layer (or Inter network layer), we need to move from the conceptual level and think about all these LANs and WANs that make up the internet. Since the story of all of them is not possible, only a small imaginary internet with some networks and some connecting devices has been shown. One of the modes of switching is circuit switching in which a circuit is created physical (or channel) between the source and destination of the message before it starts its delivery. Once the circuit is built, the entire message is transferred from the source to the destination. The source can then inform the network that the transmission has been completed, which allows the network to release all switches and use links and connect devices for another communication. Circuit switching was never implemented in the network layer, but is mainly used in the physical layer. the entire message is transmitted from the source to the destination without being divided into smaller packages. For example, in early telephone systems, in which when the caller formed the number a path was created between him and the called subscriber. So when the receiver responds to the call, a circuit is created in which voice messages can flow between the two parties, in both directions (full duplex), while all connecting devices maintain the circuit. When the caller or receiver shuts down the phone, the circuit is interrupted. Today’s telephone networks no longer use circuit switching. The network layer is designed as a network with switching with packages. This means that the source packet is divided into smaller manageable packets, commonly called datagrams. Individual datagrams are then transferred from the source to destination. The downloaded datagrams are collected at the destination before recreating the original message.

Switching with packets in the network layer:

it was originally designed as an unrelated service, but recently has

a tendency to change this way in a service with oriented connection. CO Connection Oriented: The process has three stages:

setting the connection,

data transmission,

Connectionless Oriented disconnections.

When Network Layer Internet started, it was originally designed as an unrelated service in which the network layer protocol treats each packet independently, as if each packet had no connection to any other packet. Packages in a message may or may not travel the same route to their destination. It was initially thought that the network layer service would be unrelated to make it simple. The idea was that the network layer was only responsible for sending packets from source to destination.

When the network layer provides an unrelated service, every packet that travels on the Internet is an independent entity; there is no connection between packets belonging to the same message. Switches in this type of network are called ROUTERA.A packet belonging to a message can be followed by a packet belonging to the same message, or another message. A package can be followed by a package that came from the same source or from another source. Each package is oriented based on the information found in the header of the source and destination address. The destination address determines where you need to go; the source address determines where the package comes from. The router in this case directs the packet based only on the destination address. The source address can be used to send an error message at source, if the package is discarded.

In an oriented connection service, there is a connection between all packages belonging to a message. Before all the datagrams of a message can sent, a virtual connection must be established for it determine the path for datagrams.After organizing the connection, all datagrams can follow the same way.In this type of service, the package label should not hold only the source address and destination but it should also also contain a flow label, an identifier of the virtual circuit that defines the virtual path to follow packages.

In an oriented connection service, there is a connection between it all packages belonging to a message. Although it seems that the use of the label can make the addresses useless source and destination, websites that use unrelated service in the layer of the network still hold these addresses for several reasons:

One reason is that part of the package route may still be using the unrelated service.

Another reason is that the protocol in the network layer is designed with these addresses and takes some time before can be changed.

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