lecture5: OSI Model Network layer Protocols

Layer 3 uses four basic processes:
Addressing
Encapsulation
Routing
Decapsulation



1- Addressing
First, the Network layer must provide a mechanism for addressing these end devices. If individual pieces of data are to be directed to an end device, that device must have a unique address.

In an IPv4 network, when this address is added to a device, the device is then referred to as a host.


2- Encapsulation
Second, the Network layer must provide encapsulation.
During the encapsulation process, Layer 3 receives the Layer 4 data and adds a Layer 3 header, or label, to create the Layer 3 data. When referring to the Network layer, we call this data a packet.
Layer 4 data+ Layer 3 header, or label = packet
When a packet is created, the header must contain, among other information, the destination address. The Layer 3 header also contains the source address.

3- Routing
Next, the Network layer must provide services to “direct” these packets to their destination host. The source and destination hosts are not always connected to the same network. In fact, the packet might have to travel through many different networks. Along the way, each packet must be guided through the network to reach its final destination. Intermediary devices that connect the networks are called routers. The role of the router is to select paths for and direct packets toward their destination. This process is known as routing.