Network Layer of OSI Model - MetaTutorials

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Network Layer of OSI Model

Here, we will learn about the network layer of the OSI reference model in detail with its diagram and functions. We will also discuss the protocols of Network Layer in brief. 
network-cover

What is the Network Layer of the OSI model?

The Network Layer is the third layer from the bottom in the OSI model which controls the delivery of data packets from source to destination across the different networks. 
It also finds the best routing paths for better communication in the network. It places the IP addresses of the sender and receiver in the header of each packet. 
It performs its functions on the sender as well as the receiver side. This layer divides the outgoing messages into packets for the lower level and assembles the incoming packets into messages for the higher level.  

It provides the mechanism of congestion control which occurs when a number of datagrams overload the sub-networks. In this layer, IP and IPv6 are the two main internet protocols which are used for routing and encryption techniques. 

Functions of Network Layer

The network layer performs the following various functions which are important for better communication over the different networks:

1. Internetworking: It is considered as the main function of the network layer in the OSI model which establishes the logical connection between two or more networks or network devices. 

2. Routing: The main aim of network layer is Routing. It helps the network layer by selecting the best routing path between source and destination for data communication. The process of finding an optimal path is called routing. The router is a special network device which forwards the packets to their destinations.
Following are some routing protocols which help in identifying the network paths:
a. Internet Protocol (IP)
b. Border Gateway Protocol (BGP)
c. Open Shortest Path First Protocol (OSPF)
d. Routing Information Protocol (RIP)

3. Logical Addressing: Logical Addressing is also another important function of the network layer which uniquely identifies each device on a network. This layer adds the header to each data packet which consists of the logical address of both source node and destination node. 

4. Fragmentation: This mechanism is done at the receiver side by the network layer. This technique divides the large size of the datagram into fragments so that the flow of data is not disrupted. 

Protocols of Network Layer

Following are the various protocols which help in performing the different functions of the network layer:
1. ARP
2. ICMP
3. IPV4
4. IPV6

ARP (Address Resolution Protocol)

ARP protocol is used in the network layer to link the IP address (logical address) with the MAC address (physical address). The MAC address is presented on the Network Interface Card which helps in identifying the device/node on the network. 

The main function of ARP is to convert the 32-bit IP (Internet Protocol) address into the 48-bit MAC (Media Access Control) address. The MAC (Media Access Control) address is used for communication between the two nodes on the local network. 
You can also view the ARP table in your Windows system using the following command in the command prompt:
arp-a
The above command shows the table as shown in the following screenshot:
cmd

ICMP (Internet Control Message Protocol)

ICMP protocol is a network layer protocol which is used for handling errors. It is used by the network devices such as routers which helps to diagnose network communication issues. 
This protocol sends the error messages while communicating. 

IPV4 (Internet Protocol Version 4)

IPV4 is the fourth version of Internet Protocol. The first version of IPV4 was introduced for the production of SATNET in 1982 and ARPANET in 1983. IPV4 uses a 32-bit numeric address which is expressed in dotted-decimal notation. The addresses of IPV4 are divided into the following five classes:
  • Class A,
  • Class B, 
  • Class C, 
  • Class D, and 
  • Class E. 
It is used widely in data communication over the different types of networks. In the network layer, this protocol establishes the logical connection between the network devices by providing the logical address for each device.  

IPV6 (Internet Protocol Version 4)

IPV6 is an upgraded version of IPV4. This version was introduced by the IETF in December 1998. It is used in the network layer for transmitting packets over the network. It sends and receives data in the form of packets between the two nodes. 

This is an internet layer protocol which provides end-to-end datagram communication over the multiple IP networks. Following are the three types of IPV6 addresses:
1. Unicast Addresses
2. Multicast Addresses
3. Anycast Addresses