Network Layers – TCP/IP Model

When sending data across a network, the data that is being sent needs to be packaged up and delivered to the correct device. A lot like sending a letter.

To understand this, we use the TCP/IP model to divide up the process of packing up and unpacking data.

The TCP/IP model is divided into four layers:

  1. Application
  2. Transport
  3. Internet
  4. Data Link

When sending data, the layers are followed from top to bottom. This is then reversed to unpack the data when it it received.

The Application Layer

This layer is about making data readable by humans (or taking readable data & translating it into something that can be transmitted). Protocols such as HTTP, and FTP exist at this layer.

The Transport Layer

The data is divided into packets in the transport layer. Large amounts of data can’t be sent in one large chunk, so they are divided into smaller packets (like sending several smaller parcels instead of one large one).
 
Once the data is divided up, the port number is added to the packet. A port is like a doorway into a network that identifies what kind of data is being sent.
 

The Internet Layer

When the data packets are passed to the internet layer, the sender IP address and recipient IP address is added to the port to form a socket. This is added to the header of the packet, a bit like adding the to & from address for a parcel. 
 

The Data Link Layer

When the data packets are passed to the data link layer, the sender & recipient’s MAC addresses are added to the header. This allows the individual device to be found as the IP of a device can change, whereas the MAC address doesn’t.
 

Circuit Switching & Packet Switching

 
Once the data packet is sent out onto the network, there are two ways that the data can be routed to the destination device:
 
Circuit Switching – a single route is chosen and all packets are sent along this same route one after the other until all packets have been received.
 
Packet Switching – as each packet is sent onto the network, the most efficient route is used for that particular packet. This may mean that the packets don’t arrive in order, but is often faster.

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